The Verbs Can and Could.
1. Can and Could express power or ability.
He can lift two hundred pounds.
I can’t tell you anything about that matter.
You could if you only would (= If you were willing).
2. Could is often used as a polite form.
Could you please tell me where Prof. Nalin De Silva is living now?
Could.
"3. Past Tense of Can - Auxiliary Verb.
4. Used to express possibility:
That could never be true.
5. Used to express conditional possibility or ability:
You could do it if you tried.
6. Used in making polite requests:
Could you open the door for me, please?
7. Used in asking for permission:
Could I borrow your pen?
8. Used in offering suggestions or advice:
You could ask for more information."