PAST ABILITY: COULD

Examples:

(a) When I was younger, I could run fast.

(Probable meaning: I used to be able to run fast, but now I can't run fast.)

In affirmative sentences about past ability, could usually means "used to be able to." The use of could usually indicates that the ability existed in the past but does not exist now.

(b) Tom has started an exercise program. He was able to run two miles yesterday without stopping or slowing down.

If the speaker is talking about an ability to perform an act at one particular time in the past, was/were able to can be used in affirmative sentences but not could. Could is not appropriate in (b).

--Did you read the news about the mountain climbers?

(c)  INCORRECT: They could reach the top of Mt. Everest yesterday.

(d) CORRECT:

They were able to reach the top yesterday.

They managed to reach the top yesterday.

They reached the top yesterday.

Note that (c)  is incorrect. Instead of could, the speaker needs to use were able to, managed to, or the simple past.

(e)  They couldn't reach/weren't able to reach the top yesterday.

(f) Tom couldn't run/wasn't able to run five miles yesterday.

In negative sentences, there is no difference between using could and was/were able to.

Cheers!
Naeem

Source(s):
Understanding and Using English Grammar by Betty Schrampfer Azar