The other comments here are good and they answer your question specifically, however I believe they are neglecting to point out the fact that using "could" or "can" in a polite request is not proper English. It is the same as using "would I be able" or "am I able" - which is not the correct question. For example: "Could I borrow your pen?" is the same as asking "Am I able to borrow your pen?" which does not ask the question you intend to ask. The question you want to ask is "May I borrow your pen", this is polite, formal and proper English. It should not be a question of your ability, it should be a question of the other persons willingness to allow you to borrow the pen. "Could you give me a pen?" asks if the person is able to give you a pen, of course he/she is able! Are they willing to give you pen? that is a different question... Many native English speakers use "Could I" or "Can I" in place of "May I" - so it is good to recognize and understand their meaning, however the proper and formal usage is "May I". When you want to question ones ability to perform a task it is proper to use "Can" - "Can you carry that heavy object by yourself?" - as you are questioning their ability, not asking for permission. "Can" is also appropriate when questioning your own ability: "Can I get there by turning left?" - that *is* a question of ability, so "can" is appropriate. "Could" should almost always be used in past tense, and almost always represents a potential - "that could have been a disaster", "I could have called you - if I had a phone". "I could have arrived earlier by turning left.", or "You could have used 'may' instead of 'can' in that sentence."