Can, could, and be able to are all used to talk about a person’s ability to do something. They are followed by the infinitive form of a verb.

You use can or a present form of be able to to talk about ability in the present. Can is more common, especially in speech.

1. You can all read and write.
2. I am no writer but I can draft a lecture or a report that’s reasonably lucid.
3. The rattlesnake is able to detect the presence of a small ground squirrel.

You use could or a past form of be able to to talk about ability in the past.

4. He could run faster than anyone else.
5. He was able to answer a few questions.

You use be able to and not ‘could‘ to say that someone managed to do something at a particular time.

6. After treatment he was able to return to work.
7. Dr Brancale had been able to get to Boston only late the night before.

You use ‘will’ or ‘shall’ with be able to to talk about ability in the future.

8. He will be able to provide accurate, detailed information for you.
9. One day, perhaps, I’ll be able to explain.

If you want to modify a statement about someone’s ability, you often use be able to after a modal such as ‘may’ or ‘should’. You do not use ‘can‘ or ‘could‘ after another modal.

10. We may be able to save him.
11. A man in good health should be able to go without external oxygen for at least a minute.
12. I might be able to help you.
13. You would not be able to drive to inland cities alone here.

After verbs such as ‘want’, ‘hope’, or ‘expect’ which must be followed by a ‘to’-infinitive, you use be able to and not ‘can‘ or ‘could‘.

14. I hope to be able to have wonderful touring holidays.

15. You’re foolish to expect to be able to do that.
16. As a driver you have to be able to drive, obviously.

Can and could are also used to talk about possibility. You do not use ‘be able to’ in this way.

You use could when you are saying that something is possible on a particular occasion.

17. Don’t eat it. It could be a toadstool.
18. 300,000 jobs could be lost.

You use can when you are saying that, in general, something is possible.

19. Such shifts in opinion can sometimes have a snowball effect.
20. Too much salt can be harmful to a young baby.
21. The press tell me he can appear insecure when dealing with them.

To talk about possibility in the past, you use could have followed by a past participle.

22. It could have been worse.
23. He could have been doing research on his own.

Can is also used to talk about what is allowed by rules, or what someone is willing to let another person do. When you are referring to what was allowed in the past, you use could rather than ‘can‘.

24. No student can be admitted to a first degree until he has completed full-time attendance for at least three university sessions.
25. They can leave at any time.
26. We could go to any part of the island we wanted.

Can and could are often used for asking permission or for making requests.

27. Can I take out a card please?
28. Could we put this fire on?
29. Can you send me three new men out there right now?

Could, but not ‘can’, is used for making suggestions.

30. You could phone her and ask.
31. Well, what shall we do? — You could try Ebury Street.