The semi-modal be able to appears to have two readings. On its one reading, it basically means 'have the ability to' (e.g., In her early twenties, Rebecca was able to swim across Late Balaton), whereas on its other reading, it roughly means 'have the opportunity to' (e.g., Yesterday afternoon, Rebecca was able to swim across Lake Balaton). A striking difference between these two readings is that be able to has an actuality implication in the past tense on the opportunity reading but not on the ability reading (thus the second but not the first example given above implies that Rebecca actually swam across Lake Balaton). In this paper, two previous analyses of this difference are critically reviewed and shown to have shortcomings. A new proposal is then made based on the idea that the difference between the ability and opportunity readings of be able to is due to a difference in the relative scope of tense and modality. More specifically, it is claimed that on the ability reading, tense takes scope over modality, whereas on the opportunity reading, modality takes scope over tense. This idea is formalized in a branching time framework, and it is shown that it follows that the opportunity reading of be able to in the past tense has an actuality implication, whereas the ability reading lacks one.
In WCCFL 22 Proceedings, G. Garding and M. Tsujimura (eds.), pp. 384–397. Somerville, MA: Cascadilla Press, 2003.
Paper, published version