We don’t usually use could to talk about single events that happened in the past.
When actual past achievements are mentioned, we usually use was/were able to or managed to but not could in affirmative clauses. This is because they are facts, rather than possibilities:
I was able to/managed to buy a wonderful bag to match my shoes.
Not: I could buy a wonderful bag to match my shoes.
We hired a car and we were able to/managed to drive 1,000 miles in one week.
Not: We hired a car and we could drive 1,000 miles in one week.
We use could to talk about past ability:
When I was young, I could easily touch my toes.
We use could have + -ed form to talk about possibility in the past:
I could have been a lawyer.
They could have taken a taxi home instead of walking and getting wet.
Janette couldn’t have done any better.
Guessing and predicting: couldn’t as the negative of must
When we want to guess or predict something, we use couldn’t as the negative form of must. We use couldn’t have + -ed form as the negative form of must have + -ed. Couldn’t and couldn’t have + -ed form express strong possibility:
She must have made a mistake. It couldn’t be true.