This is a good example I found online, "There is a difference between being "able to do something" and being "capable of doing something", as the sentence illustrates: Everyone is able to kill, but not everyone is capable of murder." Another example could be,"you are able to run. But are you capable of running a 5k marathon?" You are able to eat, but are you capable of eating a cat? You are able to swim, but are you capable of swimming across the Atlantic ocean?

Interesting! I m not sure there is one easy divide between the two. I can think of situations in which “able” is better suited than “capable”. I have an overall feeling that “able” has more to do with circumstances that escape our control. 1. We wouldn’t say “he’s not able to behave in public” but we would say “ he’s not capable of behaving in public” 2. In the case of an injury, a person with a broken leg would not be able to walk. And yet they are capable because they have 2 legs. 3. If I’m not capable of doing a maths problem it would be because I have never learned the theory. If I’m not able, I would rather mean that I have the capability but for some reason or circumstance I didn’t solve the maths problem. (she wasn’t able to find the answer, but she is very capable) 4. A political leader might be capable (have the capability) to maintain unity within the party but even with this capability they might not have been able to. (He wasn’t able to maintain unity within the party. Why? Probably because of circumstances that escaped his control) 5. Are you capable of keeping a secret? (Has more to do with the capability that a person has developed) It would be strange to say “Are you able to keep a secret?” But we would say “I wasn’t able to keep the secret” I am curious to see what others have to say about this!

Able to capable of Able is stronger and more strictly and mostly can describe something we can do now. like :I am able to prepare my food capable of means I am able also but mostly we use it to describe something we can do in the future potentially. like : I am capable of learning Spanish. when we want to express our ability to do something.