COULD
- It’s a modal verb.
- It is used to express ability or capability, possibility or permission in the past.
FORM
Affirmative:
- I / You / He / She / It / We / They could swim very well.
Interrogative:
- Could I / you / he / she / it / we / they swim very well?
Short answers:
- Yes, I / you / he / she / it / we / they could.
- No, I / you / he / she / it / we / they couldn't.
HAD TO
It's NOT a modal verb. It is used to express an obligation or an absence of obligation in the past.
FORM
Negative:
- I / You / He / She / It / We /They didn't have to get back home at 10.
Interrogative:
- Did I / you / he / she / it / we / they have to get back home at 10?
Short answers:
- Yes, I / you / he / she / it / we / they did.
- No, I / you / he / she / it / we / they didn't.