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I have noticed there is a way of using 'must have' to denote finished obligation in the future - somewhat akin to the 'future perfect tense'. An example of what I mean:

" I must have finished this exercise by tomorrow"

To me, "I must finish this exercise by tomorrow" would be just sufficient, but the above example is especially confusing because it seems to make perfect sense, and yet it seems to violate a rule of thumb I picked up, which is that 'must have' can only be used to express a deduction or an assumption concerning the past.

eg: "He must have dropped the pen by mistake " meaning 'I believe most certainly he did'.

All the references I checked do not cover this particular use of 'must + past-perfect'. If there were no time signifier - 'by tomorrow' in this example - I would squarely think that this is a statement about the past. So, at the risk of sounding finicky, would a native speaker ever use 'must + present perfect' in this sense?

References:
http://www.englishpage.net/showthread.php?16692-must-have-used-vs-should-have-used http://www.englishpage.com/modals/must.html

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    There is a difference between "I must have this exercise finished by tomorrow" and "I must have finished this exercise by tomorrow". I would use the first one (where finished is actually an adjective), but not the second. – Peter Shor Oct 2 '14 at 10:44
  • @PeterShor But would you say the second construction is grammatically wrong? – Arun Oct 2 '14 at 11:10
  • I would use have to and not must for this: "I have to have finished this exercise by tomorrow." In my brand of AmE, at least, I'd say must is wrong. – Peter Shor Oct 2 '14 at 12:03
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    @Arun: it's because English speakers don't use must for the future in this sense, but use have to instead. I don't know whether there's a more general grammatical rule that this is a special case of. We wouldn't use should there, either. So not *"I should have finished this exercise by tomorrow", but "I am supposed to have finished this exercise by tomorrow." – Peter Shor Oct 2 '14 at 14:05
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    @PeterShor, Fumble, It's alive and well over here. One of a few hundred thousand documents using it from the web: 15th January 2015 is the key date. By that date all the forms required must have been completed and returned to the School Administration office. PLEASE ALSO REMEMBER TO FILL IN A COMMON APPLICATION FORM for Ealing. Failure to do this will result in your child potentially not getting a place at this school. – Araucaria Oct 3 '14 at 12:06
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(1) I must have finished this exercise by tomorrow.
(2) He must have dropped the pen by mistake.

Note, first, that you are dealing here with two different senses of must. In (1), must has the deontic sense of obligation: you are required to have finished the exercise by a particular time. In (2), must has the epistemic sense of inferential necessity: you conclude that he dropped the pen by mistake.

(1) is an acceptable alternative to this:

(1a): I must finish this exercise by tomorrow.

The two versions take different perspectives. In (1a) you are speaking of an obligatory task which now, in the present, lies before you; in (1) you are speaking of an obligatory state, that of having finished the task, which will obtain in the future.

But although (1) is grammatically acceptable, most speakers would not use it to express that future obligatory state. Instead they would employ a very similar but not identical idiom:

(1b) I must have this exercise finished by tomorrow.

(1b) deploys the participle finished as an adjective modifying exercise: your obligation is seen not as that of finishing the exercise but of presenting the exercise in a finished state.

  • But 'to have something done' can have a different meaning: To get somebody else to do it. What about a different example? "I must have eaten all this food by morning" vs "I must have all this food eaten by morning". Though a weird example, the second construction conveys a different meaning namely, of having the action done by someone else. In this case, is the first construction acceptable? – Arun Oct 2 '14 at 11:09
  • @Arun Causative must have the food eaten is a different idiom, a passive version of have somebody eat the food. Confusingly, it has the exact same form as the idiom with the passive participle cast as a postposed adjectival. But it's different; and neither is equivalent to must have eaten the food. – StoneyB Oct 2 '14 at 11:17

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