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By the time they reach the station the train would have/will have left

Since it's a future action, we should be using 'will have'.

But since 'would' can also be used as a modal verb to indicate a conditional mood, 'would have' also sounds good to me, as the one who is saying cannot be completely sure of the time of the train's departure.

Please clarify.

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The rule is that you match tenses between the first verb and the auxiliary verb.

By the time they reach the station the train will have left. ['reach' and 'will' in present tense]

From the point of view of the speaker, they have not yet reached the station. This is a prediction about something that has not yet taken place.

By the time they reached the station the train would have left. ['reached' and 'would' in simple past]

From the point of view of the speaker, all this happened in the past. This is a deduction based on events and circumstances from the past.

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  • So in your second example, the train had already left but we are not sure whether they are aboard or not. Am I correct?
    – Abhi
    Oct 14 '15 at 12:39
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    No, that's not what I am saying. The second example is a typical fictional-detective type of deduction. The detective knows that the train left. S/he also knows some fact (not shown here) that makes it certain that they missed the train. The missing context is vital for example one and example two. We, the readers, haven't been let in on the secret in this example. In a real story, we would know the context. Does that explain it? If not I'll make an extended example and put it in my answer. Oct 14 '15 at 12:49
  • P.S. The word 'would' is often used as a conditional. However in this case there is no conditional - instead sentence 2 is just sentence 1 backshifted into the past. Oct 14 '15 at 12:54
  • I Understood your point. In the same context, Can you include an example using 'Would' as a conditional? Thanks.
    – Abhi
    Oct 14 '15 at 13:16
  • Conditionals: (1) If you leave home late, you will miss the train. (2) If you left home late, you would miss the train. (3) If you were to leave home late, you would miss the train. (4) Had you left home late, you would have missed the train. In all those cases there is a doubt or a hypothesis about leaving late. Oct 14 '15 at 13:25

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