To me, the following sentences sound correct.

  1. The computer crashed yesterday but I could fix it.
  2. She could pass the exam, even though she hadn't studied much.

However, my grammar book states that they aren't correct because of these reasons: can is used for GENERAL ability and be able to for a SPECIFIC ABILITY.

The book gives these examples: 3) When the computer crashed yesterday, I was able to fix it.(not 'I could fix it') 4) She was able to pass the exam, even though she hadn't studied much.(not 'she could pass')

  • Are the sentences 1 and 2 correct? If not, why and what is the difference?
  • Would 1 and 2 be sentences a native speaker would use but maybe just in informal language?

I am looking for sentences to differentiate between can (general) and be able to (specific) ability but I haven't been able to so far.

  • 3
    I agree that able to is preferable in both of these, but I wouldn't notice any oddity if somebody said them with could.
    – Colin Fine
    May 8 at 19:54
  • Nearly always, was able to strongly implies ...and did, but could often implies ...but didn't, even though it was possible. Your first example isn't particularly idiomatic anyway, and the second one is most likely to be referring to a hypothetical future (she may be able to pass) rather than something actually achieved in the past). May 8 at 20:39
  • 2
    They aren't interchangeable in the sense of permission. If I say he can go now, it means that I permit his departure. I would not convey that by he is able to go now, which would be a statement about his ability or constraints other than permission. May 8 at 22:43
  • (1) doesn't sound quite natural, but "I found that I could fix it" is better - implying that you tried and were successful. May 9 at 8:35
  • Hmm - although they have the same literal meaning, I think it's a matter of connotation. There is also "The computer crashed yesterday but I could have fixed it." which has the opposite meaning.
    – user253751
    May 9 at 9:45

3 Answers 3


When talking about the past, "could" only has the meaning "had the ability to". It never means "succeeded in doing".

For instance:

When I was a teenager, I could swim five miles in open water.

This only means that when I was a teenager, I had that ability. We can assume that I also did it -- probably several times -- but the fact of having done it is not the meaning of the sentence, only the past ability.

A clearer example that doesn't require doing anything is "could" in the sense of "had permission":

When I had my First Aid certification, I could legally teach CPR.

There's clearly no suggestion here that I ever did any teaching, just that I was legally permitted to.

In the past, it's a mistake to use "could" to talk about single accomplishments, so both of your example sentences are bad grammar, given the context of each.


"could" is one of those words that has too many different meanings, and you have to pick one based on context and gut feeling. It can be a past tense of "can" or it can be part of a conditional sentence in the present.

"I could fix the computer." can be the past tense of "I can fix the computer." with the implication that you can't fix it now. Maybe you forgot how to do it.

"I could fix the computer." can be the past tense of "I can fix the computer." with the implication that you tried and you succeeded.

"I could fix the computer." can be the second half of a conditional sentence, like "If I had a screwdriver, I could fix the computer." Even without an "if", it suggests something like "if I want to" or "if I know how". I am not sure what this is called, so I've asked another question here.

If you just write "I could fix the computer." and no context is known, it's the third meaning.

In the full sentence: "The computer crashed, but I could fix it." it is clear that the second meaning is intended. The first meaning doesn't make sense at all, and the third one doesn't make sense because "but" implies that you solved the problem. If you say "The computer crashed, and I could fix it." now it is the third meaning, although the second is also possible.

The first meaning can occur in a sentence like "Before I lost my memory, I could fix computers." To get this meaning without adding any context, we can instead say "I used to be able to fix computers."

There is also a similar sentence: "I could have fixed the computer." is counterfactual. It means that if I tried to fix the computer, I would have succeeded, but I did not try.

"She could pass the exam" is similar to "I could fix the computer" and by itself has the third meaning - there's a possibility she will pass but you're not sure about it. In context, either the second or third meaning makes sense. Either she did pass the exam, or you think it's a possibility but you're not sure.

"she hadn't studied much" is a past perfect tense and makes it clear that the whole sentence is in the past - so this sentence is actually unambiguous (meaning 2). If you change it to "she hasn't studied much" now the studying is happening in the present and the exam is going to happen in the future, so it must be meaning 3.

"be able to" gives you more tense options. Past tense is "was able to"; uncertain conditional (or whatever that is actually called!) is "might be able to". "She was able to pass the exam" or "I was able to fix the computer" is unambiguous - it is certainly the second meaning. (Unless we add some context to get the first meaning, like "Before I suffered brain damage, I was able to fix the computer")

You probably know this already, but just for completeness: in future tense, "be able to" is always used, because "can" - like most modal verbs - doesn't work in future tense. In some other languages, such as German, we can say "I will must can buy new batteries." but in English we have to say "I will need to be able to buy new batteries."


The computer crashed yesterday but I could fix it.
The computer crashed yesterday but I was able to fix it. AND

She could pass the exam, even though she hadn't studied much.
She was able to pass the exam, even though she hadn't studied much.

  • She is able to pass the exam, even if she doesn't study much.

  • She can pass the exam, even if she doesn't study much.

can = is able
could= was able

In these contexts. They both mean the same thing: have the ability to do something. So, yes, they are interchangeable.

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