Can, could, be able to (ability and possibility)

• Can/could:

 Can is a modal verb. It only has a present, past, and conditional form (but also can be used with a future meaning).
For other tenses and forms use be able to.

Example:  I can speak Spanish very well.
                She could play the violin when she was three.
                She can't come tonight. She's sick.
                They couldn't wait because they were in a hurry.

• be able to + base form

You can use be able to in the present, past, future, present perfect, and as a gerund or infinitive.
be able to in the present and past is more formal than can / could. 

Example:   I am able to accept your invitation.
                They weren't able to come.
                I'll be able to practice my English in the US.
                She has been able to speak French since she was a child.
                I'd like to be able to ski.
                I love being able to sleep late on weekends.

can, could, be able to