Both can and be able to can be used to talk about ability. In some cases they are interchangeable.
Can is used in the present tense. It is used to talk about our ability to do things.
- I can swim.
- She can speak English well.
- I can swim across that stream.
Be able to is also possible in these cases; however, it sounds a bit more formal here.
- I am able to knit. (Less natural than ‘I can knit.’)
- She is able to speak English well.
To talk about our past ability, we use could. Was/were able to is also possible.
Study the examples given below.
- She could read when she was three. OR She was able to read when she was three.
Again, in spoken English, we are less likely to use was able to.
As you can see in all of these sentences, we were talking about general ability. Things that we can or we could do at any time in the present or in the past.
To talk about things that we managed to do on specific occasions in the past, we cannot use could. Instead, we use was/were able to. The verb managed, succeeded (in…ing) are also possible in this case.
- I was able to get some really good bargains in the sale. (NOT I could get some really good bargains in the sale.)
- After climbing for several hours, we managed to get to the top of the mountain. OR After climbing for several hours, we were able to get to the top of the mountain. (NOT … we could get to the top of the mountain.)
Both could and be able to can be used to say that we were not capable of doing something on a specific occasion.
- In spite of climbing for hours, we couldn’t get to the top of the mountain. OR In spite of climbing for hours, we weren’t able to get to the top of the mountain.