Learn English – Use of ‘must have’ for obligation in the future

future-perfectmodal-verbs

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

Best Answer

(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.

Related Topic