According to this site
When we talk about ability, we mean two things.
First, we mean general ability. This is something that once you have
learned you can do any time you want, like being able to read or swim
or speak a language, for example.The other kind of ability is specific ability. This mean something
that you can or can't do in one particular situation. For example,
being able to lift something heavy, or find somewhere you are looking
for.could / couldn't (for general ability)
I could read when I was four.
My grandfather couldn't swim.
was able to / couldn't (for specific ability)
When the computer crashed yesterday, I was able to fix it.(not 'I could fix it')
I couldn't open the window.
The above explanation means "We have to use "Was able to" to express a specific ability in the past in positive sentences"
My question is that "is it ok to use "Could" to express a specific ability in the past in positive sentences"
Is it ok to say "When the computer crashed yesterday, I could fix it."
Or we must say "When the computer crashed yesterday, I was able to fix it."
Best Answer
Generally we use "was/were able to" or "managed to" instead of "could" for past archivements in affirmative sentences when we are talking about facts and not possibilities (Cambridge dictionary). This is because "could" can be ambiguos and make us guess whether it was an ability or a possibility. Here's an example:
This sentence may either mean "She could fix her computer but she didn't" or "She was able to fix her computer"
Most sources state that we shouldn't use "could" in this case if we are speaking about a past archivement. If we wish to speak about possibility in the past we should use "could have + -ed form of a verb":
This undoubtely means that she had the possibility to do it but who knows whether she did.
Notice that in negative sentences both wasn't/weren't able to and couldn't are possible for past archivements.