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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."

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    What is your question? Is it about grammer?, about usage? or the meaning that is not clear to you? What is that you don't understand? – user5267 Oct 13 '16 at 10:49
  • @AbsoluteBeginner, what's wrong? – Tom Oct 13 '16 at 11:26
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    You can certainly use could for ability in the past. For example, "Thanks to the car repair class I took, when my car broke down yesterday I could fix it." I think that site's distinction between "general ability" and "specific ability" is not 100% correct or a very good way to put it. For some cases in the past, "was able to" is more specific, because could can mean either "was able to" or "there was the possibility". For example, "She could pass the exam" leaves it ambiguous whether we mean "she was able to pass the exam" or "there was the possibility that she might pass the exam." – stangdon Oct 13 '16 at 16:41
  • That's a very helpful comment @stangdon – V.V. Oct 14 '16 at 5:33
  • @stangdon, I think your comment are not 100% correct. I found an answer ell.stackexchange.com/questions/24763/could-vs-was-able-to, was/were able to suggests that they could do it, and they did it successfully. Anh thus, we should use "was able to" in past positive sentence – Tom Oct 14 '16 at 5:46
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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:

  • She could fix her computer.

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":

  • She could have fixed her computer.

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.

  • "I wasn't able to/couldn't come because of the bad weather"
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"Was/were able to" is used in place of "could" in order to (a) disambiguate sentences, (b) put them into tenses that cannot be expressed by a single modal, or (c) change their rhythm. It so happens that in some instances, the presence or absence of these structures may coincide with a distinction between general and specific ability, but I don't think that's the key. In fact, it's easy to come up with counterexamples where both "was/were able to" and "could" can be used to represent both general and specific ability, so let's look at those first.

(1) My grandfather couldn't swim because he was afraid of the water.

(2) My grandfather couldn't swim that day because he had an appointment.

The first refers to a general ability, the second to a specific ability, but the use of "couldn't" to indicate either is valid, contrary to what the site indicates.

(3) My grandfather was able to swim because he had taken lessons as a young boy.

(4) My grandfather was able to swim that day because the lifeguard showed up.

Again, the first refers to a general ability, the second to a specific ability, but "was able to" can be used to denote either.

The phrase "could swim" can indicate either the past ("was able to swim") or the conditional ("would be able to swim"). Note that I had to use "to be able to" to differentiate between them.

One would also have to use "to be able to" in order to express the future (as that site says):

(5) I hope that some day I will be able to swim.

As for sentence rhythm, "could" is more succinct, so I would favor it in a context where that was important, such as poetry, or a fairy tale. That would be a far more important consideration to me than general vs. specific ability.

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