Interpreting Temporal Adverbials

We take for granted that sentences describe situations [2, 12]. One of the most important properties of situations are then their temporal locations, which are indicated by tense and aspect and temporal adverbials in the surface form. In [10, 22], we offered a formal theory for English tense and aspect and an algorithm that computes the temporal relationships between the situations implicitly introduced by a text. In the present paper, we propose a systematic approach to temporal adverbials, fully integrated with our tense-aspect theory and the inte~retive algorithms, using the Episodic Logic (EL) formalism [9, 11, 12, 211. 1. I N T R O D U C T I O N Previous theoretical work on temporal adverbials has mostly concentrated on adverbials specifying temporal locations (e.g., "yesterday"), durations (e.g., "for a month") and time spans (e.g., "in three hours"). It appears that interest in the first kind of adverbial originated from the desire to correct the erroneous analyses provided by Priorean tense logics, in particular, their treatment of the interaction between time adverbials and tense. The second and third kinds of adverbials were often considered in connection with the aspectual classes of the vPs or sentences those adverbials modify (e.g., durative adverbials may modify only stative sentences, whereas adverbials of time span may modify only accomplishment sentences). However, other kinds of temporal adverbials have received little attention, including ones specifying repetition: The engineer shut down the motor twice yesterday. The engine frequently broke down. The operator checked the level of oil every half hour. The inspector visits the lab every Monday. On our analysis, these sentences describe complex events, consisting of a sequence of subevents of specified types, and the given adverbials modify the structure of these complex events: the cardinality of component events ("twice"), the frequency or distribution pattern of component events ("frequently," "regularly," "every half hour," etc.), and the temporal location of cyclic events that occur synchronously with other recurrent time frames or events ("every Monday" or "every time the alarm went off' '). Other issues that deserve further investigation are the interactions between multiple temporal adverbials, and various kinds of aspectual class shift due to aspectual class constraints on the use of adverbials (occurring singly or jointly with others). The following sentences illustrate these issues. *This research was supported in part by NSF Research Grant IRI9013160 and ONR/DARPA Research Contracts No. N00014-82-K-0193 and No. N00014-92-J-1512. The authors benefited from example sentences by Greg Carlson and Phil Harrison. John ran for half an hour every morning for a month. John stepped out of his office for fifteen minutes. Mary is going to Boston for three days. Mary won the competition for four years. John saw Mary twice in two years. Our aim is to provide a uniform analysis for all kinds of temporal adverbials. Our approach is compositional in that the lexicon supplies meanings at the word level (or possibly at the morpheme level, e.g., for ' ly ' adverbs), and the meanings of adverbials are computed from the lexical entries by our GPSGlike grammar rules. The grammar rules take care of aspectual compatibility of adverbials with the VPs they modify. The resuiting indexical logical form is then "deindexed" (converted to an explicit, context-independent form) by a set of recursive rules. The resultant episodic logical form (ELF) is formally interpretable and lends itself to effective inference. We now consider the syntax and the semantics of temporal adverbials. We first show logical form representations of temporal adverbials, in both indexical and deindexed form, and how to obtain them from the surface structure, together with a brief discussion of semantics. Then, we discuss an extension of our system that accommodates aspectual class shifts to properly handle the interaction between temporal adverbials and aspectual classes. 2. SYNTAX AND SEMANTICS OF