Able is an adjective.
We use ‘able’ to mean as the model ‘can’. And the structure is be able + infinitive.
Example: Some of my friends are able to\can swim against the giant waves of the sea.
The opposite of able: Unable.
Example: He is unable to\cacan’t understand what his wife wants.
But, in some cases the use of ‘can’ is preferred:
1. In the sense of ‘know how to’:
Can you sing?
Yes, I can or I know how to sing.
(less formel, Are you able to sing?)
Point to be noted: to mean something will going to happen very soon, the use of can\could is grammatically impossible because it has no infinitive. In that case, we have to use:
Be able + infinitive
One day researchers will be able to find a cure for AIDS. (Not will can find)
He will be able to find a way for his problem.