English Grammar Reference: Ability - can, could, to be able to
We use can and could to talk about ability. We use can in the present and could in the past.
She can play the guitar. Can you speak Spanish?
They can't ride a bike.
She could read when she was three.
My grandmother could play the piano very well.
They couldn't come last year because of the bad weather.
Can and could are modal verbs and so have no infinitive, gerund or past participle. If we need to use these forms we use to be able to.
to be able to
I want to be able to speak Japanese. (NOT: I want to can speak Japanese)
I am tired of not being able to sleep. (NOT: I'm tired of not can speak Japanese)
I have been able to drive since I was 18. (NOT: I can drive since I was 18)