• ~ • 
TEACHING THE ENGLISH TENSE: 
INTEGRATING NAIVE AND FORMAL GRAMMARS IN AN INTELLIGENT 
TUTOR FOR FOREIGN LANGUAGE TEACHING 
Danilo Fum 1, Bruno Pani 2 and Carlo Tasso 2 
1 Dipartimento di Psicologia - Universit~ di Trieste, via delrUniversifi 7, 1-34123 Trieste (Italy) - 
fum@ uts882.units.infn.it.bitnet 
2 Laboratorio di Intelligenza Artificiale - Universith di Udine, via Zanon 6, 1-33100 Udine (Italy) - 
tasso@ uduniv.infn.it.bimet 
ABSTRACT 
A basic problem that must be dealt with in 
order to build an intelligent tutoring system (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. 
Two basic options are possible: (i) to use a naive or 
pedagogical grammar, comprising knowledge derived 
from textbooks and school grammars or (ii) to use 
one of the formal grammars developed by theoretical 
and computational linguists. The paper discusses the 
relationships between naive and formal grammars in 
foreign language teaching :and presents, as a case 
study, an attempt to integrate the two approaches 
within ET (English Tutor), an ITS aimed at helping 
Italian students master English verb usage. More 
particularly, the paper focuses on the possibility of 
integrating a naive grammar into a systemic 
framework. The reliability of the proposed approach 
is currently being evaluated by means of a series of 
computational experiments with the Verb 
Generation Expert of ET. 
INTRODUCTION 
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 one of the formal grammars developed 
by theoretical and computational linguists. 
Both these solutions have their shortcomings. 
Traditional grammar textbooks have serious 
drawbacks which concern both their content and the 
way it is presented to the student. The introduction 
of the notional syllabuses and the almost general 
adoption of the communicative approach 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. 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. 
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 Fum, Giangrandi, and Tasso, 1989), 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. The next section 
illustrates the systemic approach to tense developed 
by M.A.K. Halliday (1976) and C. Matthiessen 
1983, 1984). Our original contribution is then 
presented and it is shown how the naive approach 
has been integrated into a systemic framework. The 
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last section presents some evaluation criteriafor the 
present proposal. 
A NAIVE APPROACH TO 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-boxi 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 maintaining 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 order to build the Verb Generation Expert 
module, we started by examining the explanations 
about tense usage given in a set of reference 
grammars and by extracting a set of tense features 
representing the ideas and concepts which were 
utilized in providing such explanations. Then we 
described according to these features the set of 
exercises to be presented to the students. 
To give a more concrete idea of what the 
descriptions looked like, we report here the 
(simplified and partial) representation of an 
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 (cl, c2))) 
(defclause cl 
(text (when I (arrive))) 
(in-exercise exS ) 
(open-item (arrive)) 
(clause-kind (subordinate temporal)) 
(super ordinate (c2)) 
(clause-form affirmative) 
(open-item-time-interval tl ) 
(fact-kind (action single)) 
(aspect (action completed)) 
(deftemporalrelations ex5 
(before t2 now) 
(during tl t2) 
(during tl t3) 
(during t3 t2))) 
Each exercise is usually constituted by one or two 
clauses in which some of the verbs are given in the 
infinitive form and have to be conjugated into the 
appropriate tense. The exercise is.described through 
lists of attribute-values pairs, one for each clause. 
The first member of the pair indicates a tense 
feature, the second member the ivalue the feature 
receives in the clause. The exercise description 
comprizes 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 tl .. tn. In our exercise, for example, we 
find two verbs and one temporal expression, 
therefore three time intervals are utilized to describe 
the exercise. The time interval tl 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 tl t2) states that 
the time interval tl 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 (Allen, 1984). 
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 
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of production rules. In our case the following rules 
applies among others: 
IF the clause is a main clause, 
there is a subordinate temporal claus e related 
to it, 
the event time includes the event time of the 
subordinate temporal, 
TttEN set the reference time to the event time of 
the temporal subordinate clause. 
The rule asserts that the main clause c2: 
Yesterday Tom (talk) on the telephone 
has as its reference time tl, i.e., the time interval 
represented by the event time of the subordinate 
temporal c1: 
when I (arrive). 
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 cl and c2, respectively: 
IF the clause describes a past event, 
the reference time is past, 
the event is completed 
?'HEN 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 
TttEN 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 thing that needs to be done at this 
point is to conjugate 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 Me 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 
ease 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 linfitations, 
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. 
THE SYSTEMIC APPROACH TO 
TENSE SELECTION 
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. It is 
sometimes claimed, in fact, 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 
151 , 
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." 
(Matthiessen, 1984, 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 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. 
COMBINING THE NAIVE AND 
SYSTEMIC APPROACHES 
The 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 
didactic 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 that concerns us in the ET project, is 
well documented in the systemic approach 
through papers by Halliday himself (Halliday, 
1976) and, from a computational point of view, 
by Matthiessen (1983, 1984); 
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, Halliday, Mclntosh and Stevrens, 
1964); 
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 
(Patten and Richie, 1987) and to express it into 
- 152 - 
a notation that is compatible with the 
formalisms, like functional unification 
grammar, currently used in computational 
linguistics (Kasper, 1987). 
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 the 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. 
The most important problem that has been dealt 
with in applying the systemic model to the 
representation of the grammatical knowledge for the 
new Verb Generation expert has been that of 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 anw.cedents. 
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. His procr~dure 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 en,,ironment 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 ~mantic and pragmatic 
factors play a covert role in the identification of the 
comparison time the choosers receive as ioput. 
According to our point of view, it is possible 
to make explicit the criteria underlying the 
determination of the tense and build eognitively 
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 comprizes 
those features which express the temporal relation 
among the time intervals occurring in the exercise 
sentence; the second class comprizes 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 
furst 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 a previous section: 
Yesterday,when I (arrive), Tom (talk) on the 
telephone. 
The solution to this exercise requires the use of the 
(simple) past for the fhrst 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 
- 153 - 
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 
TtlEN 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. 
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 MaclI, 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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