Ability or potentian (2): can, could, be able to

Can is used to indicate (1) the possesion of ability in general, or (2) the ability in particular circumstances, to perform the activity indicated by the main verb:   1 He can speak German fluently.   2 I can (or could) give him an answer (e.g.) now, later, tomorrow. In sentence 1, can forms part of a statement having general current validity whereas in 2, it refer to an ability existing in particular cercumstances at the present or futuretime indicated. It is important to recognize this distinction between ability in general and ability in specific circumstances. It is also important to note that both sentences refer to a potentioal performance of the actoin mentioned (speaking German, giving an answer), not to an actual performance.   1a He could speak German fluently (e.g.) when he was younger. This sentence represents the equivalent of sentence 1in the chronological past. It refers to the possesion of the ability to speak German, not to an actual performance of speaking.   2a I could have given him an answer (e.g.) yesterday. This is the exact equivalent in past time of sentence 2, and means 'I was in a position to give him an answer' (ability in specific circumstances at a specific time). Again, it does not refer to an actual performance; indeed, it implies that I did not give him an answer. Such sentences suggest a conditional idea:   I could have given him an answer (e.g.) if he had asked me. If we wish to refer to an actual performance, we use a form of be able to, as in sentence 2b below:   2b I was able to give him an answer (e.g.) yesterday. 3 I Can see are doing. could hear were saying. Verbs like see, hear, understand, etc. come into a special category. The ability to see and the performance of seeing are inseparable ( I can see = I am seeing, I could see = I was seeing), and in this case the use of could is possible when referring to an actual performance in past time. The negative form couldn't necessarily indicates non-performance of an action, and may always be used to refer to past time:   4 he can't speak German fluently.   4a He couldn't speak German fluently (e.g.) when I knew him.   5 I can't give hima an answer (e.g.) right now.   5a I couldn't give hima an answer (e.g.) yesterday. In all the examples so far, we could substitute a form of be able to for can or could, but we tend to use the latter (shorter) forms where possible.

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