Past Ability

Grammar explanation

General ability

We usually use could or couldn't to talk about general abilities in the past.

She could paint before she started school.
I couldn't cook until I went to university.
When I lived next to the pool, I could go swimming every day.

Ability on one occasion – successful

When we talk about achieving something on a specific occasion in the past, we use was/were able to (= had the ability to) and managed to (= succeeded in doing something difficult).

The burglar was able to get in through the bathroom window.
The burglar managed to get in through the bathroom window even though it was locked.

Could is not usually correct when we're talking about ability at a specific moment in the past.

Ability on one occasion – unsuccessful

When we talk about a specific occasion when someone didn't have the ability to do something, we can use wasn't/weren't able to, didn't manage to or couldn't.

The speaker wasn't able to attend the conference due to illness.
She couldn't watch the match because she was working.
They worked on it for months but they didn't manage to find a solution.

Note that wasn't/weren't able to is more formal than couldn't, while didn't manage to emphasises that the thing was difficult to do.