Could and to be able to

We use could to talk about general abilty in the past and we use to be able to or managed to to talk about a specific situation in the past.

Examples

So we say

I could run fast when I was a child.

because it is a general fact, but we say

I was able to run five miles without stopping last night.
or
I managed to run five miles without stopping.

because we are talking about a specific time in the past.

Other examples

I could play football well I was a boy.
Last night I managed to score four goals.

I could study all night when I was a student.
I wasn’t able to concentrate on my studies last night.