a pt. of can 1 .

(used to express possibility)

I wonder who that could be at the door. That couldn't be true.

(used to express conditional possibility or ability)

You could do it if you tried.

(used in making polite requests)

Could you open the door for me, please?

(used in asking for permission)

Could I borrow your pen?

(used in offering suggestions or advice)

You could write and ask for more information. You could at least have called me.