In speaking about the past, 'could' tells us about our ability to to something, but does not necessarily say that we did it; 'was/were able to' says that we had the ability and did it. So, "I could swim well yesterday" is not impossible, but it is far less likely than "I was able to swim well yesterday".
There are several exceptions to this, two being:
1. In the negative, lack of ability to do something implies that you didn't do it, so there is very little, if any, difference between 'couldn't' and wasn't/weren't able to.'
2. For verbs denoting the senses, such as see, hear, smell, taste, feel, we very often use 'can' when we would use a progressive form with other verbs:
I am looking at the top of the mountain now and, oh yes - I can see the climbers.
The past of this is 'could'.
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