Asking permission - can/could
You want to borrow friend's laptop.
(can) + answer the question
- Can I borrow your laptop?
- Yes, no problem
You want to have another piece of cake.
(could) + your possible answer
Could I have another piece of cake, please?
Yes, sure. / Yes, of course.
You want to open the door.
(could) + your possible answer
Could I open the door?
No, please don't.