The expression "be able to" means the same as "can", but it is used differently.

BE ABLE TO

The special verb CAN and the normal verb BE ABLE TO have the same meaning.

- She's able to speak German = She can speak German
- I'm not able to do that = I can't do that
- Are you able to drive a bus? = Can you drive a bus?

The verb CAN is "defective", it is only used in the present, past and conditional:

present You can dance very well
past I could climb trees when I was a child
conditional I could help you if you let me

So for the rest of tenses we can only use the form BE ABLE TO.

future If you practise a lot, you will be able to do it  (not: you will can do it)
infinitive I want to be able to finish in time
present perfect She has never been able to sit for more than 5 minutes
etc.