Could have vs would have

If you had told me, I would have used the other route.
If you had told me, I could have used the other route.

Would here denotes a certain consequence of the unreal condition: no question about it, I would have used the other route.

Could denotes a possible consequence of the unreal condition: if you had told me, I might still have taken this route but I would have had the choice of taking the other route.

If you had told^{Past Irrealis} me, I would^{Present Irrealis} use the other route.
If you tell^{Present Realis} me, I would^{Present Irrealis} use the other route.

These are not quite impossible, but the circumstances under which either would be acceptable are very unlikely to arise. Ordinarily these forms would be understood as having the tense/mode significances I have noted in superscript, and in most cases the two verbs, the one in the IF clause and the one in the THEN clause, should have the same tense and mode.^†

^† This is not always the case in conditional clauses involving logical inference; but I do not think either of these sentences can be read as inferential.

StoneyB on hiatus

Posted 2014-05-07T15:17:54.043

Reputation: 176 469

What about "I might have used the other route."? Is it the same as "I could have used the other route."? – Kinzle B – 2014-05-08T09:17:53.853

@ZhanlongZheng Yes. – StoneyB on hiatus – 2014-05-08T10:21:36.267

Besides, doesn't "could have used" signal the ability? Here it should be equivalent to "would have been able to use", which I think has nothing to do with a possible consequence as "might have used" signals. – Kinzle B – 2014-05-08T10:27:07.413

@ZhanlongZheng But in practical terms they amount to the same thing: because you failed to inform me, my 'capability' remained merely potential, I had no reason to actualize it. Can implicates do - in fact, simple past could usually means did: "I found I could do anything I put my mind to.* Irrealis could implicates did not. – StoneyB on hiatus – 2014-05-08T10:45:30.847

Welcome to ELU! Please make an effort to observe ordinary rules of punctuation and capitalization. And you can make your question easier for people to read and answer by employing the available formatting options. See THIS.

– StoneyB on hiatus – 2014-05-07T15:24:06.033

Go back in time where these sentences are turned into present tense. This may give you some hint: *If you tell me, I will use the other route.* and *If you tell me, I can use the other route.* – Maulik V – 2014-05-07T15:26:48.663

Could have vs would have

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