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. 

Wah How to use "Must/Might/May/Can't be"

When you are sure something is true, you can say "must be". 
        He is speaking Japanese, He must be Japanese. (I am sure he is Japanese.)
        She must be a police officer.(I think she is a police officer.)
        This must be the way to the park. (I am certain this is the way to the park.)

If you are quite sure something may be true, you can say "may/might be".
        He is so late at the meeting, the traffic might be bad.
        John is not at the office now, he may be taking the day off.
        I am not sure but she might be older than me.

And If you want to say something you think it is unlikely, you can say "can't be".
        He is very old, he can't be the student.
        Sarah is at the hospital, she can't be working today.
        This restaurant is very delicious, it can't be going out of business.

Kenny Chesney - Must Be Something I Missed
"There must be something I missed
I wake up in the morning just making a fist
I don't call it living, I just exist
There must be something I missed..."