modal verb
1
BAD : As I've already mentioned, I could learn a great deal during the two years I spent in England.
GOOD : As I've already mentioned, I was able to learn a great deal during the two years I spent in England.
BAD : By reading quickly, I could finish the book before the library closed.
GOOD : By reading quickly, I managed to finish the book before the library closed.
◆◆◆
When talking about a skill or general ability in the past, use could or be able to : 'By the time she was four, she could/was able to swim the whole length of the pool.'
When talking about a single event in the past, we usually use be able to or manage (NOT could ): 'Luckily, we managed to get there before the shops closed.' 'Were you able to start the car?'
See also ABLE 2 (↑ able )
2
SEE POSSIBLE 2