When you are "Capable" you have the ability or what is necessary to do a job or help When you are "Able" it means that you can...and have all the qualification to do a job or help A person that is capable usually has to work on something a little more but they can manage A person that is able usually knows the ways around something

It is possible to be capable of doing something but not able to do something. "capable" is often used with tasks we have the ability/skill/talent to complete. "able" can have the same meaning but also can be controlled by other factors. It's best to explain with examples: I am capable of walking home by myself but I'm not able to because my mum said I need to take a taxi instead. I am capable of beating you at tennis but I'm not able to beat you today as I'm busy. I could replace "capable" with "able", but double "able" would make the sentences weirder.