You need to be clear what function "could" has in each sentence. a) present tense - similar to "can" but indicates doubt e.g. Can you go to Indonesia tomorrow? I could go but I don't really want to change my plans. b) past ability - e.g. I could play football when I was younger but I can't any more. c) conditional ability/possibility (would be able to) If I went to London, I could visit the Queen.

Good question. There is a very subtle difference between the two. Could is used for something 'possible', and can is for something 'definite'. For example: "Can you help me?" vs "Could you help me?" If the answer is yes, then the difference in the meaning is reflected in the answers below. "Can you help me?" Yes = I can definitely help you. "Could you help me?" Yes = It is possible to help you, but it is not clear that it is going to happen. However, the two are generally interchangeable as when most people use "could" they mean "can". There has been many a fight between many young siblings because of the minor difference in meaning: "I COULD let you borrow my Nintendo Gameboy". "You said I CAN!" "No I said I could!" I hope this makes it a bit clearer for you.