As far as I know, you use was/were able to or manage (but not could) in a positive statement when you are saying that something was possible on a particular occasion in the past: I was able to/managed to find some useful books in the library. But it's not a mistake to use 'could' if it's a negative statement, am I right? Eg., I couldn't find some useful books in the library.
Thanks
Top answer
Yes, you are right, except that you need to say 'I couldn't find any useful books in the library'. Rover
— Rover_KE
Yes, you are right, except that you need to say 'I couldn't find any useful books in the library'.
Rover
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