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Can or be able to?

"Be able to" and "can" often have a very similar meaning. However it is not always possible to use "be able to" instead of "can".

1. "Can" should be used if we talk about a general present ability, e.g. I can swim. I can play the piano. I can speak Spanish. (=I know how to do it.)

2. "Can" should be used with verbs describing senses (see, hear, smell, taste) and also with verbs describing thinking (uderstand, remember, decide, believe), e.g. I can smell smoke in this room. We can believe her.

3. "Can" should be used to talk about something that is happening at the moment of speaking, e.g. Look at me! I can stand on my hands!

4. "Can" should be used before passive voice, e.g. Your luggage can be left here.

5. If we talk about a single action (achievement) from the past we use "was/were able to", not "can" or "could", e.g. I was able to get the tickets, although it cost me a fortune.

6. "Be able to" is used when "can" is not grammatically possible just because it's a modal verb:
a) after another modal verb (will, would, shall, must, should, could, may, might)
b) after semi-modals (used to, ought to, need, dare, had better, have to, have got to)
c) in perfect tenses (present perfect, past perfect and future perfect)
d) after "to" with any possible meaning or function

Examples: I hope I will be able to swim soon. You might be able to persuade him to do it. I used to be able to run 100 meters in under 12 seconds. She has been able to do it for many years. He had been able to help us until we moved to another country. I would like to be able to see you next week. He came to the meeting to be able to explain himself.