Modal verbs for Ability: can, could, be able to, managed to

+We can use 'can' to talk about abilities in the present or future.
-She can speak Spanish very well.
-Can you come with us on Saturday?

+We can use 'could' to talk about general abilities in the past.
-I could run much faster when I was younger.
-She could play the violin when she was a child.

+'Can' and 'be able to' have the same meaning, so you can usually use either verb without any difference in meaning:
-She can speak Spanish very well.
= She’s able to speak Spanish very well.
-I could run much faster when i was younger.
= I was able to run much faster when I was younger.

+However, 'can' and 'could' can’t be used in all verb forms or tenses. There are many cases like this. In those cases, we use 'be able to' instead.
-He’s been able to swim since he was two years old. (NOT He’s canned swim… .)
-I like being able to choose my own working hours. (NOT I like canning… .)
-I want to be able to speak more than one language. (NOT I want to can … .)

+Generally, if you have a choice, it’s better to use 'can/could', because it makes your sentence simpler.

We sometimes choose to use 'be able to', even when 'can/could' are possible, because it sounds more formal.

To talk about an ability in a specific situation in the past, use 'was/were able to' or 'managed to' (NOT 'could').
We could say:
-The traffic was really bad, but we were able to catch our train.
-The traffic was really bad, but we managed to catch our train.
We can’t say:
-The traffic was really bad, but we could catch our train.

+However, in negative sentences, there’s no difference between 'could' and 'be able to', whether the situation is general or specific:
-It was hard work, and we couldn’t finish everything on time.
-It was hard work, and we weren’t able to finish everything on time.