Wah How to use "Be able to"

We use be able to talk about ability. 
We sometimes use it instead of can and could.

• I cannot visit my parents. = I am not able to visit my parents.

We can use be able to in many different forms.

Future Tense

• Tomorrow, he will be able to join our party.  Tomorrow, he will can join our party.  is incorrect.
• Will you be able to go to the park with me next week?
• I am afraid I won't be able to go with you.

Present Perfect Tense

• Simon has been always able to remember his students' name.  Simon has been always can remember his students' name.  is incorrect.
• I have never been able to swim.

Past Tense

• I was able to solve the question this morning. (= I could solve the question this morning.)
• I was able to explain to her.

Infinitive Forms

• John is going to be able to help me.
• You have to be able to speak English if you want to get this job.
• You must be able to understand it. It's very important.

I would like to be able to play the piano.