you can check the Alex's video about this article. http://www.engvid.com/expressing-ability-can-could-be-able-to/

Natalia is correct, very nice. Note that even though the affirmative "could" is not used in reference to specific situations, the negative form "couldn't" is permissible to talk about a specific situation. In addition, the word "could(n't)" could be used in a conditional, meaning "would(n't) be able to."

We use could for GENERAL ability (способность в общем). But if you want to say that somebody did something in a SPECIFIC situation, use 'was/were able to' or 'managed to '. F.e. In childhood I could swim (Т.е. здесь говорится о моей общей способности, что я умела плавать) We had a competition, and I managed to swim (здесь же говорится о конкретной ситуации - У нас было соревнование, и я смогла проплыть.)

Could implies that you can do something, but the action remains to be completed I could drink 5 glasses of beer, but i dont want to. It could happen - it's possible (but we dont know for sure) Was able to/ managed to implies that the action was completed . I was able to/ managed to drink 5 glasses of beer without problems!

What is the diffrence ... In the futur we can only use able to Both can and be able to can be used to talk about ability Can is probably more common in spoken English, simply because it’s shorter.