Hi,
"Capable" is stronger than "able to" or "can". "Are you capable of doing this?" You're talking to the person like it's a very difficult task or you're angry or sarcastic toward the person. Maybe the person is injured and will have a very difficult time of doing the task. Maybe the task seems impossible! "Are you able to do this?" That's weird to say and hear, however it is more formal than "Can you do this?". It's better to say, "Can you do this?" in an informal setting. The task is not as difficult. I just want to know if you can do it or not. "Is he capable of understanding?" is much stronger and has more emphasis than, "Is he able to understand?". "Is he capable of understanding-because he must be stupid or deaf or something... ("Can he understand" sounds better and is less formal than "Is he able to understand" and is normal, calm speech.
So...
Are you capable of doing this? (very difficult task, maybe speaker is sarcastic, very formal speech, negative or angry question with right tone of voice)
Are you able to do this? (somewhat difficult task, asking politley if task is possible, formal speech)
Can you do this? (may be a command (Do it!), informal, not as polite as "Could you do this?")