POLITE QUESTIONS: MAY I, COULD I, CAN I

(a) May I please borrow your pen? yes
(b) Could I please borrow your pen? yes. of course
(c) Can I please borrow your pen? yes certainly.
Sure (informal)
Okay. (informal)
Uh-Huh. (meaning yes)
I'm sorry, but I need to use it myself.
People use may I, could I, and can I to ask polite questions. The questions ask for someone's permission or agreement. (a), (b), and (c) have basically the same meaning.
Note: can I is less formal than may I and could I.
Please can come at the end of the question: may I borrow your pen, please?
Please can be omitted from the question: may I borrow your pen?