Picking Reference Events from Tense A Formal, Implement able Theory of English Tense-Aspect Semantics Trees

A formal solution requires (a) a well-defined mapping from a subset of English covering the most common tense-aspect constructions (including embedded tensed clauses) to a formal meaning representation, and (b) a well-defined denotational semantics for the meaning representation, which accords with speakers' intuitions about the original text. We propose a simple structure called a tense tree (or a set of connected tense trees) as a basis for interpreting tense-aspect constructions and some time adverbials in a context-dependent way. The de-indexicalization process simultaneously transforms tense trees and logical forms, the former in accord with simple recursion equations and the latter in accord with formal equivalences between context-indexed and context-independent logical forms. The rules have been implemented, and yield meaning representations in a formal episodic logic for narrative understanding. I. I n t r o d u c t i o n Narratives describe and relate episodes (events, situations, eventualities). The episodes successively introduced appear to "line up" in systematic ways as a function of the text structure, with tense and aspect playing a crucial role. (1-2) illustrate this familiar phenomenon: (1) John grinned at Mary. She frowned. (2) John knocked, but soon realized Mary had left. Each of the verbs in (1) appears to introduce an episode; the past inflection places them before the utterance event (or speech time), and the surface ordering suggests temporal sequencing, and probably a causal connection. In (2) we appear to have at least three episodes, a "knocking," a "realizing" and a "leaving." The past perfect auxiliary relates the "leaving" to a past episode serving as reference point. (This reference point seems closely correlated with the "realizing" episode, but on our theory is not identical with it.) As well, the adverb soon implicitly references and relates the "knocking" and "realizing" episodes. The problem of automatically extracting these relationships is complicated by a number of subtle issues: • How many episodes (events, situations, etc.) does a sentence implicitly introduce? We remarked that the introduction of episodes appears correlated with