Both can and be able to can be used to talk about ability and permission. In some cases, both structures are possible. However, in some other cases only one of these two structures are used.
- She can knit. OR She is able to knit.
- He can work on a computer. OR He is able to work on a computer.
Of course, in the sentences given below, the structure with can sounds more natural and hence is more common.
Here are the rules regarding the correct usage of can and be able to.
You can use ‘can’ or ‘be able to’ to express present or future ability or possibility. Note that in the future tense, we use will be able to.
- She can speak four languages.
- He will be able to finish it on time.
- They will be able to come.
- I can come on Sunday.
- I will be able to come tomorrow.
Past
To talk about general ability that existed in the past, we can use could.
- She could read when she was three.
- We could go swimming whenever we wanted to.
We do not normally use could to say that you were able to do something on one particular occasion. In this case, was / were able to is used.
- The trains were late but I was still able to reach on time. (More natural than ‘The trains were late but I could still reach on time.’)
- We were able to find our way. (NOT We could find our way.)
In the negative, both could and be able to are possible.
- We couldn’t find our way. OR We were not able to find our way.