赵微微, Su.Ki. and MoiraWendy are correct, but "can" sometimes means "do you have the ability to do something" and "could" sometimes means "is it possible that you may be able to do something in the future (and the sentence usually includes some conditions that would make it possible or impossible". For example: Q - Richard, can you play the piano? A - No, I cannot, because I never took any lessons....... / Q - Richard, could you play the piano? A - Yes, but only if I took lessons first. Is my comment 'clear as mud'?