Naive vs. Formal Grammars: A case
for integration in the design of a foreign language tutor
Danilo Fum 1,
Bruno Pani 2,
and Carlo Tasso 2
1 Dipartimento di Psicologia - Università di Trieste, via dell'Università 7, I-34123 Trieste (Italy) - fum@uts882.units.infn.it.bitnet
2 Laboratorio di Intelligenza Artificiale
- Università di Udine, via Zanon 6, I-33100 Udine (Italy)
- tasso@uduniv.infn.it.bitnet
Abstract:
The relationship between formal and naive (i.e., used for didactic
purposes) grammars in foreign language teaching is dealt with
in the paper which presents, as a case study, an attempt to integrate
the two approaches within an intelligent tutoring system. This
work has been carried on in the framework of the ET (English
Tutor) project whose long term goal is the development of
a tutoring system aimed at helping Italian students master English
verb tenses. Within the project, a prototype system based on
a naive approach to the grammar of tense has been built. The experimentation
performed with the prototype provided the motivation for a critical
re-evaluation and revision of some of the assumptions which it
was grounded upon. The possibility of integrating some naive intuitions
into a systemic representation of grammatical knowledge is discussed
in the paper, and a new version of the domain expert module exploiting
the systemic approach to tense selection is illustrated.
Keywords: intelligent
tutoring systems, foreign language teaching, systemic grammar.
1. Introduction
One of the most distinguishing features of an intelligent tutoring system (ITS) - at least in comparison with the traditional computer-aided instruction programs - is constituted by the explicit representation of the domain knowledge it utilizes. This knowledge does not represent only a description of the concepts and skills to be taught, but often makes up a runnable model which allows the system to generate answers and explanations, and to execute the tasks and exercises presented to the student. As for systems working in the domain of foreign languages, this knowledge is represented basically by grammatical knowledge. The so-called domain expert module of such systems represents therefore a model of the grammatical competence the student is intended to acquire.
A problem that must be dealt with in order to build an ITS in the domain of foreign language teaching is that of establishing what kind of grammatical knowledge has to be included in the Domain Expert module. At first sight, two distinct options are possible:
a) to utilize the knowledge contained in textbooks and school grammars;
b) to adopt some of the formal grammars developed by theoretical and computational linguists.
Both these solutions have their shortcomings.
A lot has been said about the grammar which is taught in schools and the textbooks utilized for such teaching. Traditional grammar textbooks (see the discussion, for example, in [2, 4, 25]) have serious drawbacks which concern both their content and the way it is presented to the student (conceptual descriptions framed on classic languages, definitions and explanations that are often incoherent and grounded upon heterogeneous criteria, overly focused attention to written language, morphologic problems and 'exceptions', etc.). The introduction of the notional syllabuses and the almost general adoption of the communicative approach [14, 28, 29] have somehow changed the general attitude and the strategies utilized in foreign language teaching, but even the grammars that follow these methodologies do not overcome the most severe limitation of what we call the naive approach to the representation of linguistic knowledge, i.e., the incapacity to provide a global and coherent model of language.
The formal grammars developed by linguists, on the other hand, show their shortcomings when we try to use them directly for didactic purposes. The point is that these grammars have been developed to pursue goals that are different from those of school grammars. Theoretical linguists are in fact interested in providing mathematically well defined descriptions of a language which capture the competence of a native speaker. Computational linguists are interested in discovering computationally effective models of the processes that allow the speaker to utter or to understand a sentence in that language. Both kinds of descriptions are mostly centered around the syntactic properties of the language. Considered from the point of view of a foreign language teacher, these formal descriptions are generally useless since the (meta)language in which they are framed and the concepts which they are grounded upon are different from those utilized in daily teaching. Moreover, syntactic descriptions are not enough: learning a language means not only to become aware of the set of structures available in the language but also to realize how and when such structures could (and should) be used, and which contribution they provide to the communication process [15].
Such a situation, in which scientific research and teaching practice go along parallel roads, without mutual exchanges and influences, presents several disadvantages. On one hand, the naive grammars utilized in schools support an empirical kind of teaching that is not rooted in linguistics research. The theoretical basis of such a teaching is in fact generally fragile, the concepts and the principles it relies upon are often unsystematic and badly defined. On the other hand, the elegant formal descriptions developed by linguists are not immediately usable in practice. The main issue concerning the relationship between formal and naive grammars is still that pointed out by Saporta: "A central question in the application of linguistics to the teaching of foreign languages involves the conversion of a scientific grammar into a pedagogical grammar" [26]. Such a conversion, as far as the ITS development is concerned, requires the following steps:
- to choose, among the different formal linguistic theories, the one which appears particularly promising from a pedagogic point of view;
- to develop, within the chosen theoretical framework, an explicit description of the language subset we are interested in teaching;
- to fit such a description into a computationally adequate and pedagogically reliable form.
The relationship between formal and naive
grammars in foreign language teaching is dealt with in this paper
which presents, as a case study, an attempt to integrate the two
approaches within an intelligent tutoring system. The work has
been carried on in the framework of the ET (English Tutor)
project whose long term goal is the development of a tutoring
system aimed at helping Italian students master English verb tenses.
Within this project, ET-1, a prototype system based on a naive
approach to the grammar of tense (described in [5]), has been
built. The experimentation performed with ET-1 provided the motivation
for a critical re-evaluation and revision of some of the assumptions
which the prototype was grounded upon. The possibility of formulating
some naive intuitions into a systemic representation of grammatical
knowledge is discussed in the paper and a new version of the domain
expert module exploiting the systemic approach to tense selection
is illustrated. The following section presents our previous naive
approach to a grammar of English verb tense, describes how the
grammatical knowledge has been utilized by the domain expert module
of ET-1, and clarifies why such an approach has been found in
the long run unsatisfactory. Section 3 illustrates the reasons
why, among the different formal descriptions, the systemic approach
to tense developed by Halliday [8] and Matthiessen [16,17]
has been chosen, while section 4 illustrates its main features.
Section 5 presents our original contribution showing how the naive
approach has been integrated into a systemic framework and which
changes to the original formulation have been made. Section 6
presents some evaluation criteria for the present proposal and
suggests guidelines for future research.
2. A Naive Approach to Verb Tense
Selection
The basic goal pursued in constructing the domain expert module - called Verb Generation Expert - of ET-1 has been that of building a glass-box model of the competence underlying the choice and conjugation of an English verb tense. One of the main concerns in designing the knowledge base for this module has been that of using the same (meta)language and the same concepts found in the naive grammars which we referred to [3, 7, 19, 22, 27].
The following explanation - concerning some
subtle meaning distinctions that are possible to express through
the use of the present continuous tense - is typical of what we
mean by a naive approach.
164. Other possible uses of present continuous:
a. With a point in time to indicate an action which begins before this point and probably continues after it [...]
The present continuous is rarely used in this way except in descriptions of daily routine and in dramatic narrative [...]
b. With
always for a frequently repeated action, often one which
annoys the speaker or seems unreasonable to him [...] Sometimes,
especially when used with the first person, always with
the continuous tense implies that the action is accidental, while
always with a simple tense would imply a deliberate action.
[27 , pg. 95]
This explanation utilizes some intuitive and informal concepts that exploit the student analysis capabilities and rely on his linguistic insights. In fact, it is difficult to realize how it is possible to formally establish that an action constitutes a daily routine, that it can annoy the speaker or that the text represents a dramatic narrative. These ideas, however difficult to express in a computational form they may be, have, however, deep pedagogic value since they form part of the teachers metalanguage and are utilized to talk about and to explain the object language.
It is possible to provide other examples of
naive explanations that involve intuitive concepts like the formal
or informal register:
Be going + infinitive is commonly used to express the future in informal speech [30, pg. 46]
Be to + infinitive
can be used to refer to a future plan or programme. This is a
fairly formal way of expressing the future and is used more often
in written than in spoken English [30, pg. 61]
or that refer to some complex psychological
states:
The problems of future tense in English
involve not only aspects of activity and time but also mood, i.e.
the certainty, hesitation, willingness, determination, hope, sense
of obligation or of prohibition, or pure neutrality, with which
the speaker views what is to come [...] Each expression of futurity
is often complicated by subjective attitudes on the part of the
speaker and his audience ... [3, pg. 96]
In designing the Verb Generation Expert for ET-1 we wanted to maintain the wealth of ideas and intuitions existing in the naive account of tenses while developing at the same time a computationally tractable model of the tense selection process.
In ET-1, the Verb Generation Expert module [6] is devoted to generating the right answers to the exercises presented to the student. Usually the exercises are constituted by one or two sentences in which some of the verbs (open items) are given in the infinitive form and have to be conjugated into the appropriate tense. The Verb Generation Expert is therefore able:
- to select the grammatical tense for each open item of the exercise in order to correctly describe the status of the world the sentence is intended to represent;
- to appropriately conjugate the verb according to the chosen tense.
Besides these basic functionalities, the tutoring environment in which the domain expert operates imposes a further requirement, i.e., the Verb Generation Expert should be able:
- to explain to the student how the solution has been found, which kind of knowledge has been utilized and how.
In order to build the Verb Generation Expert, we started by examining the explanations about tense usage given in the reference grammars and by extracting a set of tense features representing the ideas and concepts which were utilized in providing such explanations. Some of these features were strictly syntactic and referred, for instance, to the clause kind (main or secondary clause at various levels of subordination) or the clause mood (affirmative, negative, interrogative, interrogative-negative). Other features dealt with the functional properties of the sentence constituents. Still others referred to semantic and contextual aspects, such as the kind of action described in the clause, the temporal length of the action described in the clause, the register (formal vs. informal) of the utterance, etc. Each exercise was then described according to these tense features.
To give a more concrete idea of what the descriptions
looked like, we report here the (simplified) representation of
the exercise:
Yesterday, when I (arrive), Tom (talk)
on the telephone.
(defexercise ex5
(text (Yesterday when I (arrive) Tom (talk)
on the telephone))
(structure ex5 (clauses (c1, c2)))
(defclause c1
(text (when I (arrive)))
(in-exercise ex5)
(open-item (arrive))
(clause-kind (subordinate temporal))
(superordinate (c2))
(clause-form affirmative)
(open-item-time-interval t1)
(fact-kind (action single))
(aspect (action completed))
(defclause c2
(text (Yesterday Tom (talk) on the telephone))
(in-exercise ex5)
(open-item (talk))
(clause-kind (main))
(subordinate (c1))
(previous-coordinate nil)
(clause-form affirmative)
(open-item-time-interval t3)
(time-expression ((yesterday) t2))
(fact-kind (action single))
(aspect (action uncompleted)))
(deftemporalrelations ex5
(before t2 now)
(during t1 t2)
(during t1 t3)
(during t3 t2)))
An exercise is described basically through lists of attribute-values pairs, one for each exercise clause. The first member of the pair indicates a tense feature, the second member the value the feature receives in the clause. The exercise description comprises also a list of temporal relations expressing the relationships that exist between the time intervals mentioned in the sentence. These time intervals are associated with the situations (states and/or events) described by the sentence verbs and with the temporal expressions occurring in the sentence, and are represented through the symbols t1 .. tn. In our exercise, for example, we find two verbs (both open items) and one temporal expression, therefore three time intervals are utilized to describe the exercise. The time interval t1 is associated with the state indicated by the verb to arrive, the time interval t2 with the temporal expression yesterday, and so on. The temporal relations specify the relationships existing between these intervals so, for example, (during t1 t2) states that the time interval t1 is included within the interval t2 : the verb to arrive indicates thus an action that happens within the time interval represented by t2 (i.e., yesterday). A special time interval is represented by the symbol now which stands for the speaking time, i.e., the interval during which the sentence is being uttered.
Since the number of the potential temporal relations holding between the time intervals contained in the sentence could be quite large, only the relations directly derivable from the exercise text are explicitly represented in the description. At the beginning of its operation, the Verb Generation Expert deduces therefore from the stated temporal relation all the possible relations holding between the various time intervals. In doing this, it applies a set of inference rules that implement a reduced version of Allen's temporal logic [1]. Thus, for example, since t2 (i.e., yesterday) by definition comes before the speaking time now (i.e., (before t2, now)) and since it is asserted in the exercise description that (during t1 t2) , it is possible to infer that (before t1 now ), i.e., that the action described by the verb to tell refers to a time interval (event time) that comes before the speaking time. The same rule allows the inference that (before t3 now) i.e., that the action indicated by the verb (talk) happened in the past too.
In order to be able to choose the tense for
a sentence clause containing an open item, it is generally necessary
to know not only the relation between the time in which the sentence
is uttered and the time of the events described in the sentence,
but also the relation which holds between the event time and the
so called reference time, i.e., the interval of time the
situation described in the clause refers to. So, for example,
in the sentence:
By the end of next month I shall have finished
my thesis.
the speaking time is now, the event time is given by the time interval associated with the action to finish the thesis and the reference time is constituted by the time interval indicated with by the end of next month.
In some clauses the reference time may be absent and, in such cases, the only temporal relationship involved in the choice of the tense is that which holds between the speaking time and the event time.
In the following operation step, the Verb
Generation Expert computes the reference time (if it exists)
for every exercise clause through a series of production rules.
In our case the following rules applies:
IF the clause is a main clause,
there is a subordinate temporal clause related to it,
the event time includes the event time of the subordinate temporal,
THEN set the reference time to the event
time of the temporal subordinate clause.
and
IF the clause is a subordinate temporal,
the event time is included in the event time of the main clause,
there is at least a temporal expression in the related main clause,
THEN set the reference time to the most
specific time expression of the main clause.
The first rule asserts that the main clause
c2:
Yesterday Tom (talk) on the telephone
has as its reference time t1, i.e.,
the time interval represented by the event time of the subordinate
temporal c1:
when I (arrive)
This clause, on the other hand, has as its
reference time the time interval represented by the most specific
time expression of the related main clause c2. In our case
it is t2, i.e., yesterday , whereas in a sentence
like
Yesterday, after the Geography lesson,
Tom (talk) on the telephone
the reference time would be represented by the temporal expression after the Geography lesson.
Once the reference times for the exercise
clauses have been computed, it is possible to choose the tense
for each open item. To do this, a set of tense selection rules
are used. The antecedent of these rules is constituted by some
conditions concerning the tense features that must hold in the
clause description, while the consequent indicates the tense that
has to be assigned to the open item. In our example, the following
rules are utilized for the clauses c1 and c2, respectively:
IF the clause describes a past event,
the reference time is past,
the event is completed
THEN use the simple past tense.
and
IF the clause describes a past event,
the reference time, if defined, is past and it is included in the event time,
the event is not completed
THEN use the past continuous.
According to our rules, therefore, the tense that is chosen for to arrive is the simple past while to talk has to be conjugated into the past continuous.
The last step to be carried out at this point is the conjugation of the verbs into the chosen tenses. For the regular verbs a set of conjugation rules are exploited, whereas the conjugation of the irregular forms is performed by a simple dictionary look up.
Leaving aside some computational complexities deriving from the need of drawing the logical temporal inferences and of computing the reference time for each exercise clause, the process performed by the Verb Generation Expert relies on the same concepts and rules described in the naive grammars. However, the adoption of the naive approach has its problems, as we realized by experimenting with the prototype.
First of all, the translation of a naive grammar into a computationally suitable form is not straightforward. The explanations given by the naive grammars - the 'tense selection rules' that are derived from the textbooks - are in fact incomplete and even inconsistent. As a result, ET-1 was sometimes incapable of solving a given exercise since the rules of the grammar did not cover that particular case. In other instances we found the opposite to be true, i.e., we obtained multiple incompatible solutions for the same exercise since several rules could be legitimately applied to the case at hand. The computational application of the naive grammars, in other words, disclosed some deficiencies and incongruities that went unnoticed in the original formulation.
Second, the informal concepts used in the naive grammars and utilized in ET-1 to express the tense features have generally no well stated definition. This means that it is difficult to attribute unequivocally the value to the temporal features describing an exercise since a lot is left to the insight of the exercise coder. Different implementers can thus describe the same exercise in a different way and obtain therefore different, often incompatible, solutions.
From the experimentation performed with the
prototype, and from an analysis of its limitations, the need of
a theoretically sound formulation of the grammatical knowledge,
keeping as far as possible the 'cognitive transparency' of the
naive grammar, has arisen.
3. The Reasons of a Systemic Choice
Systemic grammar, a theory originally formulated by the linguist M.A.K. Halliday (for a general introduction, see [9] and also [13, 18) has been chosen as the formal grammar to be utilized by the Verb Generation Expert module of the new version of ET. Systemic theory puts a strong emphasis on the functional aspects of the language: grammar, according to this approach, is a description of the resources which a speaker can rely upon in order to convey a given meaning. The most important concept of systemic grammar is that of system i.e., a mutually exclusive set of classes which express the options from which a speaker can choose in planning an utterance. Thus, for example, the system for the clause mood presents the speaker with a choice between an indicative and an imperative clause: the two options are alternative. Or, to make another example, in deciding the person of a pronoun, the speaker can select among first, second, and third. A particular choice, however, is not always applicable; thus some sort of context must be introduced to determine when a given set of choices becomes relevant. In systemic grammar, context is defined in terms of other choices: systems, in other words, are hierarchically ordered according to entry conditions in the so called system networks. To take our example a step further, if the speaker chooses to utter an indicative clause, he can select between a declarative and an interrogative. If the latter is the case, he can decide between a wh- or a polar (yes/no) kind of question.
Systemic grammar interprets a language as being a huge network of meaning potentials. The task of the grammar is to specify, through the net of systems, the total grid of options available to a speaker of the language. The introduction in the systemic grammar of this semantic functional dimension reflects the thesis that language is a social activity taking place in a situational context and fulfilling a number of social (meta)functions. More particularly, language can be used to communicate ideas (and the logical relationships existing among them), to organize the text into a relevant and coherent discourse fitting into the communicative situation, to establish and maintain social relations, to influence people's behavior, to express the speaker's feelings, attitudes and opinions, etc. A systemic description would include an understanding of the way grammatical structures and constituents contribute to one or more of these (meta)functions.
This view of grammar as a set of resources from which to choose and the focus on the social role of language are two of the reasons that support systemic grammar as a candidate formalism for pedagogic utilization. It is evident, in fact, that the notion of choice, the concept on which such grammars are based, is more familiar to teachers and students than other abstract principles (e.g., unification) which other formalisms rely upon. The emphasis on the functional organization of the language - how it presents speakers with systems of meaningful options as a basis for communication - makes systemic grammar in keeping with modern approaches to language teaching. But there are other reasons that support such a choice. Among these we mention:
- the fact that the grammar of tense, the subset of language dealt with in the ET project, is well documented in the systemic approach through papers by Halliday himself [8] and, from a computational point of view, by Matthiessen [16, 17];
- the interest shown by Halliday for the issues related to teaching, and the fact that much of his writing has been aimed at this topic (see, for example, [10]);
- the fact that the systemic approach provides us not only with a static description of linguistic structures but, especially in the computational application of Matthiessen, with a runnable model of language;
- the fact that it is possible to translate the systemic approach into rigorously formal terms [21] and to express it into a notation that is compatible with the formalisms, like functional unification grammar, currently used in computational linguistics [12].
For all these reasons, the systemic approach
has been judged particularly suitable to serve as the conceptual
ground for an intelligent tutoring system devoted to foreign language
teaching. At the best of our knowledge, this is the first time
that a (subset of) a systemic grammar has been utilized as part
of an ITS.
4. The Systemic Approach to Tense
Selection
In this section the systemic approach to the problem of tense selection is briefly described. What follows is based on the work of Halliday [8] and, in particular, of Matthiessen [16, 17].
According to the systemic approach, two assumptions are made concerning the grammar of the English tense. These assumptions are:
a) Tense opposition: the tense in English is considered as a three term opposition. From a linguistic point of view, it is an opposition of past vs. present vs. future; from a semantic point of view, as we will see below, it is interpretable as a precedence relation between two temporal variables.
b) Seriality: complex tense combinations can be constructed by repeatedly selecting among the three term opposition.
As far as the first assumption is concerned, it should be noted that not all the linguists agree with the idea of English as a three-tense language. Some scholars claim in fact [20, 24] that in English it is possible to distinguish only between present and past, the future being a modal form of the present.
The second assumption reduces the process of tense selection to a series of iterative choices concerning the three term option. In other words, a tense combination like 'is going to have built' is chosen by picking up the first time (primary tense) the present, then (secondary tense) the future and finally (ternary tense) the past. The name for a tense combination in the systemic approach is determined by considering the inverted order of the choices: in our case the tense combination is a past in future in present.
An important
point concerns which possible tense combinations are allowed.
It should be noted that, according to Halliday, up to quinary
tenses (like: 'will have been going to have been taking':
a present in past in future in past in past) are
admissible in English. Some tense combinations, however, are not
allowed; in English, for instance, there is no future in future
in present and the following sentence is considered ungrammatical:
* Henry is going to be going to cook dinner
The restrictions that the English grammar puts
on the possible tense combinations are called 'stop rules' by
Halliday and can thus be paraphrased:
1. The present can occur only at the ends of the tense sequence (as a beginning or final choice).
2. Except in the last and penultimate place, the same tense cannot occur in two consecutive positions.
3. The future can occur only once, apart from
the last position.
These rules define whether a tense combination is legitimate but they do not indicate how a given tense combination is selected. To this end a significant contribution has been given by Matthiessen with his notion of chooser. To each option concerning the tense, and represented in the grammar through a system, Matthiessen assigns a chooser "that states how the selection among the options specified is controlled. A chooser is a procedure that consists of steps that ascertain conceptual distinctions and make grammatical choices according to the conceptual distinctions." [17, pg. 1].
According to this point of view, a verb tense essentially indicates the temporal relation which holds between the speaking time and the event time, and the tense selection process is determined by such a relation. More particularly, for each iteration step, the choosers take into account a relation of precedence (anteriority) - that we symbolize through '<' - between two different temporal variables (let us call them Tx and Ty ) and:
- if Tx come after Ty (Ty < Tx), then the past is chosen;
- if Tx comes before Ty (Tx < Ty), then the future is chosen;
- if none of the above alternatives holds, then the chosen tense is present.
The tense selection process can be expressed
through the following algorithm:
Tx := Ts;
repeat
find the value for the comparison time Tc;
Ty := Tc;
if Ty < Tx
then Tense := past
else if Tx < Ty
then Tense := future
else Tense := present;
Tx := Ty
until Ty matches Te
The process, in other words, starts by setting the time variable Tx to the speaking time Ts and by looking for the comparison time Tc, i.e., the time interval the speaking time is related to. This is the time that is assigned as a value to Ty. At this point it is possible to choose the primary tense according to the relation which holds between Tx (= Ts) and Ty (=Tc). If the comparison time matches the event time Te, then the temporal relation holding between Ts and Te has been found and the resulting tense combination consists only of a primary tense (a simple present or a simple past or a simple future). If, on the other hand, the comparison time is different from the event time, the process cannot terminate since no temporal relationship has been established between the speaking time and the event time. A new iteration cycle starts by assigning the old Tc to Tx and by looking for a new comparison time Tc to be assigned to Ty. The choice of the secondary tense is made again according to the relation holding between Tx and Ty and the process terminates if Tc matches Te. If this is not the case, the process goes on according to the same modalities with a tertiary, quaternary or quinary tense, until a link between the speaking time and the event time will be found.
From the above description it is evident how complex tenses arise from the pairwise comparison of times. Each order of tense (primary, secondary, tertiary, etc.) has its own reference time, the Tx of each Tx-Ty pair. The first reference time is Ts, the speaking time, while each subsequent reference time is the comparison time related to the reference time of the previous tense. Given a reference time Tx, it is necessary to establish what time is to be related to it through a tense selection, i.e. it is necessary to establish the Tc to be assigned as a value to Ty. This is a question that ultimately concerns text planning: the choice of the temporal relations among different times depends in fact on the meaning a given utterance is intended to convey.
The identification of the comparison time for each iteration step is based on an interaction between the tense choosers and the so called environment, i.e., the non-linguistic part of the system "where conceptual information resides, for instance the knowledge base and text plans" [16, pg. 3]. It is useful to think of this interaction as a dialogue, with the choosers presenting inquiries and the environment responding. The choosers act thus as an interface through which semantic/pragmatic considerations can be brought to bear on choices in the grammar. It is the task of each chooser to select grammatical features according to the conceptual distinctions that exist in the environment. The identification of the comparison time for each iteration step is based therefore on the answer the environment provides in response to a specific chooser question concerning what time is to be directly related to Tx. This inquiry is obviously compulsory for primary tense. Since there is always a primary tense in a temporal clause, a time to be related to Ts has to be determined. The situation is different for the other tenses (i.e., secondary, tertiary, etc.) being that the identification of a new comparison time is dependent on whether a link between the speaking time and the event time has already been established or not.
Two points should be emphasized at the end of this description of the systemic grammar of tense. The first concerns the fact that, in the Matthiessen's approach [16, 17], the identification of the comparison time - which represents, according to our point of view, the most critical step in the tense selection process - is a process that falls outside the grammar since it is ultimately based on the speaker's plans and goals and takes into account those semantic/pragmatic factors that form part of the environment. This will represent, as we shall see below, a critical issue in our systemic treatment of tense.
The second point concerns the semantics of
tense. According to Matthiessen, the verb tense expresses a temporal
relationship between two times: the speaking time and the event
time. These two times are directly linked in the case of a primary
tense; they are connected through the mediation of one, two, three,
or four intervening times in the case of secondary, ternary, quaternary
and quinary tenses, respectively. The fact that no tense exists
beyond the quinary depends on reasons of meaning and text planning,
not on grammatical motives. As Matthiessen puts it,"constraints
on communicative complexity lead to constraints on semantic and
grammatical complexity" [17, pg. 131]. Other approaches (among
these the classic account of [23], see also [11]) claim that the
temporal structure is restricted to three elements, i.e, the speaking
time, the event time, and one reference time. According
to the latter point of view, tense is always a function of three
variables (Ts, Te, Tr) whereas in the former it is an
n-ary function (with n varying from 2 to 6).
5. Combining the Naive and Systemic
Approaches
Two main problems have been dealt with in applying the systemic model to the representation of the grammatical knowledge for the new Verb Generation Expert:
- the first deals with the treatment of the tense features utilized in the naive grammar according to the systemic approach;
- the second concerns the construction of
the choosers for selecting the appropriate tense combination.
5.1 The treatment of the tense features
In the first version of the ET prototype, the process of tense selection was performed through a group of production rules exploiting a set of (sometimes fuzzily defined) tense features derived from the naive grammars. Some of these features (mainly of morphological and syntactic nature) are indeed compatible with the systemic approach and it has been easy to put them directly into the systemic network. So, for example, the features referring to the clause kind, the clause mood, the case roles etc. are immediately translatable into systemic terms. Other features, however, refer to properties that depend on word sense and discourse context. To establish, for example, the time required by an event or whether an action is performed voluntarily or whether it is habitual demands some general world knowledge and the knowledge of the context in which the communication occurs. It is difficult therefore to assign a value to the features which are not directly expressible in systemic terms but which are, however, explicitly mentioned in the naive grammar and utilized by the tense selection rules.
In the new version of the Verb Generation Expert these features appear as dictionary attributes. In other words, the dictionary reports the value these features assume for each lexical item. Thus the information that to be born and to die are actions that cannot be iterated, that inadvertently and thoughtless express the idea that something is done without any conscious intention, or that daddy is a word that is used only in an informal context whereas Sir represents a formal way to address someone is contained in the dictionary.
An interesting point concerns the way these
features influence each other. The tense features associated,
for example, to a verb definition can thus be reinforced or contradicted
by the occurrence in the sentence of other words (for instance
adjectives or adverbs) whose features agree with or differ from
those of the verb. Let us consider for example the following
sentence:
(Due to a timer malfunctioning) the light
flashed all through the night.
The verb flash generally expresses an action that is short, almost instantaneous; the event described in the sentence is, however, long since the real temporal length of the action is expressed not by the feature event-duration of the verb but by the temporal expression all through the night. Instead of a single instantaneous event, therefore, the sentence indicates an iterated action that occurs within a rather extended period of time.
To manage these cases, a series of heuristics
have been developed which state how the tense features of the
modifiers take over the features of the modified words. As it
is the case for all the heuristics, ours also have no absolute
value. They work satisfactorily for the exercises the system
is capable of solving even if it is always possible to find some
counterexamples in which the setting of the values to the tense
features is wrong. The rationale behind the heuristics is different,
however. They represent an attempt to implement the hypothesis
that a speaker is capable of assigning the right value to the
tense features by relying on the sense of the words which occur
in the sentence and by computing the meaning the words assume
in the context of discourse. By using these heuristics the assignment
of the values to the tense features is made computationally feasible,
and the values of the features irreducible to a systemic approach
results not from a subjective judgment of the exercise implementer,
but from an effective procedure - even if it is based on heuristic
criteria.
5.2 The construction of the tense choosers
A second, and more important problem, concerns the construction of the tense determination rules (choosers) capable of establishing in a cognitively transparent way (i.e., using as much as possible the ideas and concepts of the naive approach) a tense combination according to the assumptions of seriality and opposition of the systemic approach. Adopting these assumptions led to a complete change of the original verb generation strategy which was based on the direct choice, in a single step without iteration, of the verb tense according to a heterogeneous set of features taken into account by the rule antecedents.
Solving the problem of tense determination according to the systemic approach requires finding the solution to the following subproblems:
- how to choose the tense in each iteration step;
- how to stop the iteration process.
We have examined in a previous section Matthiessen's proposal [16, 17]. His procedure for choosing the tense in each step is based on successive comparisons between the reference and the comparison time, while the termination procedure is based on a match between the current comparison time and the event time.
The burden of the whole process falls primarily on the identification, in each iteration step, of the appropriate comparison time, and this is performed through a dialogue between the choosers and an environment representing the semantic and pragmatic factors influencing the choice of the tense. These factors are, however, hidden from the choosers which simply receive from the environment the answers to their inquiries. In other words, the choosers work by exploiting only the temporal relations between the different times, while the semantic and pragmatic factors play a covert role in the identification of the comparison time the choosers receive as input.
According to our point of view, it is possible to make explicit the criteria underlying the determination of the tense and build cognitively adequate choosers that utilize them directly. In our new approach, this is done by dividing the features used by the tense selection rules of the naive grammar into two classes: the first class comprises those features which express the temporal relations among the time intervals occurring in the exercise sentence; the second class comprises the features of morphological, syntactic, semantic, and pragmatic information. These two classes play a different role in determining a tense combination. More particularly, the temporal features are utilized by a first type of chooser which discriminates in each iteration step between past vs. present vs. future (selection choosers). The remaining features are utilized by a different category of choosers whose task is to establish whether the tense selection process should be iterated or not (termination choosers). Differently from Matthiessen, however, the decision of the termination choosers is not based on a simple matching procedure which checks whether a link between the speaking time and the event time has already been established.This decision instead exploits a series of more complex factors through which the criteria underlying the determination of a tense combination in a given language are made explicit.
Let us clarify this new approach, which integrates
the systemic treatment of tense with the use of the features found
in the naive grammars, by reconsidering, as an example, the exercise
presented in section 2:
Yesterday, when I (arrive), Tom (talk)
on the telephone.
The solution to this exercise requires the use of the (simple) past for the first verb and the present in past (or past continuous) for the second open item. While in the previous version of the Verb Generation Expert these solutions were chosen through appropriate rules that directly established the correct tenses, according to the systemic theory the tense determination process is iterative. In other words, the first tense is determined by choosing past for the primary tense and halting the process. The second tense is determined by choosing past as the primary tense, iterating the selection process for the secondary tense in which present is chosen, and then stopping.
In order to perform such a process, both the
selection and termination choosers are needed. In our case, the
following chooser applies:
IF the clause describes a past event,
the reference time is past,
THEN set the primary tense to past.
This chooser is very similar to the naive tense
selection rule utilized to establish that a given verb should
be conjugated into the simple past: it has been obtained, in fact,
by withdrawing from that rule the condition concerning an aspectual
feature (the event is completed). The selection choosers work
thus by exploiting only the temporal features, and the choice
among past vs. present vs. future is performed by taking into
account only the temporal relations among the states and events
described in the sentence. This chooser allows the identification
of past as the primary tense for both the open items of the exercise.
As for the first open item, after the primary tense has been selected,
a termination chooser can be applied:
IF the primary tense is past
the clause contains an explicit time expression
the action described in the clause has been completed
THEN stop.
The termination choosers work by exploiting features of morphological, syntactic, semantic, and pragmatic information. While the tense selection choosers take into account the temporal aspects of the tense determination process, the termination choosers represent an interface between the tense system of a particular language and the relationships among the states or events described in a sentence the speaker intends to convey through the usage of a given tense.
In our case, the first condition determines the applicability of the chooser (it represents one of the termination choosers for the (simple) past), while the remaining conditions put forward two of the criteria that establish when the simple past represents a necessary and sufficient tense combination for expressing a given meaning: i.e.. when the action described in the sentence has been completed in the past at a definite time.
No termination choosers are applicable to
the second open item which therefore resorts to a secondary tense
selection. The following selection chooser applies:
IF the clause has a definite reference time,
the event time is equal to or includes the reference time
THEN set the secondary tense to present.
As a result of the action performed by the
chooser, the secondary tense is set to the present. After the
secondary tense has been determined, the following, very simple,
termination chooser applies:
IF the primary tense is present,
the secondary tense is past
THEN stop.
According to the systemic grammar of English
tense, in fact, no further tenses are possible after a combination
of present in past has been chosen.
6. An Evaluation of the Integrated
Approach and Future Developments
In the paper a new approach to the problem of determining the tense combination for an English sentence has been proposed with integrates the treatment of tense in a systemic grammar with the naive approach in school grammars. The systemic theory provides general assumptions (i.e. three-tense opposition and seriality) which the tense selection process relies upon, while the naive features provide the criteria for terminating the selection process. The integration of the naive approach into a systemic framework can be evaluated according to three different perspectives:
- Computational. How effective is the proposed theory? What is its coverage? How general is it?
- Pedagogical. Is it possible to utilize such a theory to really teach the English verbs? How efficient is such an approach in comparison with the traditional one?
- Psychological. To which extent does the serial theory of time mirror the real processes that occur in the mind of a speaker ?
The ongoing research tries to answer these
questions. A series of computational experiments with the new
Verb Generation Expert, implemented in PROLOG on a MacII, is under
way with the goal of establishing the reliability of the proposed
approach. The construction of a new Tutor aimed at teaching the
serial theory of time is under development. Finally, a series
of psychological experiments concerning the cognitive validity
of the systemic treatment of tense are being planned.
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