ABLE

We use able especially in the structure be able + infinitive. This often has the same meaning as can. There is a negative form unable.
Some people are able to I can walk on their hands.
I am unable to / can't understand what she wants. Can is preferred in the sense of 'know how to', and in expressions like can see, can hear, etc.
Can you knit? (More natural than Are you able to knit?)
I can see a ship. (More natural than I am able to see a ship.)
Be able is used in cases (e.g. future, present perfect) where can/could is not grammatically possible.
One day scientists will be able to find a cure for cancer. (*not ... will can find...)
What have you been able to find out? (*not What have you could... ?)
I might be able to help you. (*not I might can...)
Able is not usually followed by passive infinitives.
He can't be understood. (*not He's not able to be understood.)