First of all, both are (adj)
You can use (able):
1/ when you can do some thing
2/ We always put "To" after (Able) followed by a base form verb
e.g. I am able to learn English.
You can use (capable)
1/ When you are capable of doing something
2/ It always comes before Gerund (ING form)
e.g. I am capable of taking care of my daughter
This difference is only in terms of grammar but there is another obvious difference in usage, it is as follows:
You can use it as an adjective before nouns:
e.g. You are a capable learner