Meaning of could

could

1. a pt. of.

1. (used to express possibility): I wonder who that could be at the door. That couldn't be true.
2. (used to express conditional possibility or ability): You could do it if you tried.
3. (used in making polite requests): Could you open the door for me, please?
4. (used in asking for permission): Could I borrow your pen?
5. (used in offering suggestions or advice): You could write and ask for more information. You could at least have called me.