A Simplified Theory of Tense Representations and Constraints on Their Composition

A SIMPLIFIED THEORY OF TENSE REPRESENTATIONS AND CONSTRAINTS ON THEIR COMPOSITION Michael R. Brent MIT Artificial Intelligence Lab 545 Technology Square Cambridge, MA 02139 michael@ai.mit.edu ABSTRACT This paper proposes a set of representations for tenses and a set of constraints on how they can be com- bined in adjunct clauses. The semantics we propose ex- plains the possible meanings of tenses in a variety of sen- tential contexts. It also supports an elegant constraint on tense combination in adjunct clauses. These semantic representations provide insights into the interpretations of tenses, and the constraints provide a source of syntac- tic disambiguation that has not previously been demon- strated. We demonstrate an implemented disambiguator for a certain class of three-clause sentences based on our theory. 1 Introduction This paper proposes a set of representations for tenses and a set of constraints on how they can be combined. These representations provide insights into the interpre- tation of tenses, and the constraints provide a source of syntactic disambiguation that has not previously been demonstrated. The sentences investigated in this paper contain multiple clauses connected by temporal/causal con- nectives, words like once, by the time, when, and be- fore. (1) shows that the tenses of multi-clause sentences affect their acceptability. This raises several important *when } (1) a. * Rachel won the game *once Jon *before arrives here answer these questions. Specifically, they provide explanations in terms of the meanings of the tenses. We propose an explanatory theory and demonstrate an im- plementation which successfully disambiguates a class of three-clause sentences. The issues raised by (1) are significant for compu- tational linguistics on several accounts. First, an under- standing of the constraints on tense combinations can be used to support syntactic disambiguation. For example, consider the alternative parses shown textually in (2) and graphically in Figure -1. The first parse in both (2) a. oK b. * [s Jon will learn [s that he won s/ when Rachel arrivess] Read as: When Rachel arrives, Jon will learn that he won Jon will learn [s that he won when Rachel arrives s/ Read as: Jon will learn that, when Rachel arrives, he won (2) and Figure -1, where the adjunct clause starting with when is attached high, is fine; the second, where it is at- tached low, is unacceptable. Figure -1 demonstrates our parser discriminating between the acceptable and unac- ceptable parses of (2). The details of the representation cannot be understood until later, but it can be seen that different compositions of the tenses in the two parses result in marking the top node of the second parse as bad. The contrast between example (2) and example (3) shows that whether the preferred attachment depends on the tenses of the clauses. Examples (2) and (3) show b. OK Rachel will win the game Jon arrives when } once before questions. Which tense combinations are acceptable and which are not? Why do they have the status they do? How can observations like (1) be used to …

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