CAN or COULD or BE ABLE TO
Can is used to say that somebody knows how to do
something: Can you play the piano? / I can cook very well. / I can’t do it.
Can or be able to are used to say that
something is possible or that somebody has the opportuniy to do something: Can
you / are you able to come on Saturday?
You use be able to to form
the future and the present perfect tenses: You’ll be able to get a taxi
outside the station. / I haven’t been able to get much work done today.
Could is used to talk about what someone was
generally able to do in the past: Our daughter could walk when she was nine
months old.
You use was/were able to (but
not could) when you are saying that something was possible on a
particular occasion in the past: I was able to find some useful books in the
library. In negative sentences, could not can also be used: We
weren’t able to / couldn’t get there in time.
Could have is used when you are saying that it
was possible for somebody to do something in the past but they did not try: I
could have won the game but decided to let her win.
Adapted from Oxford Advanced Learner`s Dictionary
