These modal auxilary verbs present a few difficulties to learners as past ability and past actions are not expressed in the same way in the affirmative. In the negative form they are interchangeable even though they may have subtle differences according to the context.
Notice the similarities:

Could/was or were able to = had the ability or capacity to do something
was or were able to = managed to do something or succeeded in doing something
Couldn’t/wasn’t able to do something = had the ability or managed to do something

Be careful!

Could followed by certain sensory verbs can also express unique actions: see, hear, taste, feel, smell, understand, remember and guess:

I could smell something burning when I walked into the house. I could understand everything she said.
And we can use could to talk about one occasion with negative adverbs such as hardly or only: She could hardly believe her eyes. I could only get six eggs.

Could can also sometimes refer to one occasion in subordinate clauses:

I’m so glad that you could come.

Listen to Alex, an ESL instructor, explain the differences and then do the quiz at Debbys Corner.

Quick Test (answers are printed below): Choose the best answer
1) George has travelled a lot. He __________________ speak four languages.
[ ]could
[ ]able to
[ ]can

2) I used to __________________ stand on my head but I can’t do it now.
[ ]be able to
[ ]can
[ ]could

3) When Tim was 16, he was a fast runner. He __________________ run 100 metres in 11.
[ ]could
[ ]can
[ ]able to

4) Sandra __________________ drive but she hasn’t got a car.
[ ]could
[ ]can
[ ]able to

5) Ask Catherine about your problem. She might __________________ help you.
[ ]be able to
[ ]could
[ ]can

6) I was feeling sick yesterday. I __________________ eat anything.
[ ]can’t
[ ]haven’t been able to
[ ]couldn’t

7) Sue wasn’t at home when I phoned but I __________________ contact her at her office.
[ ]can
[ ]was able to
[ ]could

8) When we went into the house, we __________________ smell burning.
[ ]could
[ ]can
[ ]were able to

9) Will she __________________ cope with the work?
[ ]could
[ ]be able to
[ ]can

10) I can’t see you on Friday but I __________________ meet you on Saturday evening.
[ ]can
[ ]am able to
[ ]could


1. can
2. be able to
3. could
4. can
5. be able to
6. couldn’t
7. was able to
8. could
9. be able to
10. can

Click on the following links for additional practice:

  1. From Englisch-Hilfen: http://www.englisch-hilfen.de/en/exercises/modals/can.htm 
  2. AnglaisFacile (Rules and Exercise) : http://www.anglaisfacile.com/exercices/exercice-anglais-2/exercice-anglais-14361.php
  3. AnglaisFacile: http://www.anglaisfacile.com/exercices/exercice-anglais-2/exercice-anglais-19753.php
  4. English Learning Platform: http://englishbee.net/grammar/can_beable/exer1/can_able_to.htm
  5. ESL Quizzes: http://esl.about.com/library/quiz/blgrquiz_canbeable.htm
  6. English Club Quiz: http://www.englishclub.com/grammar/verbs-modals_can_quiz.htm

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