For the simple past (and also positive) Could, would there possibly be any sentence that you just can't simply replace Could with Was able to or Managed to?

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    How much wood could a woodchucker chuck if a woodchuck could chuck wood? – jmoreno Dec 7 '18 at 2:23
  • @jmoreno I guess that's the conditional Could rather than simple past. – dolco Dec 7 '18 at 2:26
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    @dolco No, jmoreno means that you can't replace "could" with "able to" because A) it's too many syllables, and B) it doesn't rhyme with "wood". So it would mess up the silly little poem, How much wood would a woodchuck be able to chuck if a woodchuck was able to chuck wood :-) – Andrew Dec 7 '18 at 3:00

In general terms, such a switch should be okay. But here are two caveats:

  1. It's not always easy to tell if what you have on your hands is a simple past without sufficient context, and as you know, you would completely change the meaning if you replaced the wrong kind of "could".

  2. The precise meaning of "can/could" does not have perfect overlap with the notion of possibility. Sometimes it expresses permission.

— When you worked at Acme, were you given frequent smoke breaks?

— Yeah. I could step outside and have a smoke whenever I wanted.

If you replace this "could" with "was able to", you get a slightly different meaning than if you replace it with "was allowed to". Of course, there is overlap (you're able because you're allowed), but it depends on how nuanced you want your meaning to be. No doubt there are other cases where the difference betwen possibility and permission is starker.

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  • Thank you for the answer :) I have one more question though. How would it sound to natives' ears if I rewrite "I did all I could" into "I did all I was able to"? – dolco Dec 7 '18 at 11:45
  • @dolco Good question. I answered this in terms of meaning and grammar, but not style. "Was able to" will usually not sound as nice — longer, harder to say — but sometimes it's good to avoid ambiguity with the hypothetical "could". For your specific example, I recommend sticking with "could". No one would think it was wrong, but "all I could" is so common it's almost a fixed expression. Here's an Ngram so you can compare. – Luke Sawczak Dec 7 '18 at 14:39

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