The verbs can and could are both helping or auxiliary verbs. Use can and could to talk about people’s ability to do things.
Can and could are used with the pronouns I, you, he, she, it, we and they, and with singular or plural nouns.
Could is the past tense of can.
He can run faster than Arthur.
She cannot afford such an expensive ring.
I’m full. I can’t eat any more.
Can you help me?
Can I come with you?
I knew you could do it if you tried.
She could not come because she was ill.
Miss Lee said we could go home early.
All the King’s men could not put Humpty
Dumpty together again.
Kaynak: Basic English Grammer Book1