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Is the question "Must you have eaten all the food" correct when used in the past obligatory sense?

The best examples I can find are quite ambiguous.

"Must He have been less than perfectly kind to one of His creatures? I do not think it can reasonably be argued that in such a case God must have wronged one of His creatures." -Kopelman, ‎L.M. and Moskop, J.C.; Ethics and Mental Retardation, 1984 [pg. 130]

If you change the question to read as follows it seems to share a similar intent:

"Did He have to be less than perfectly kind to one of His creatures? I do not think it can reasonably be argued that in such a case God must have wronged one of His creatures."

I am looking for solid references or consensus as to why "much" can't be used in the past deontic sense.

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    Yes but once we invent time travel I look forward to English getting a past imperative. What is your question?
    – JeffSahol
    Commented Sep 10, 2013 at 0:56
  • In that case, check out the answer to this question: english.stackexchange.com/questions/51023/…
    – JeffSahol
    Commented Sep 10, 2013 at 1:20
  • @TomBurchell Please click the "edit" link on your question to edit it and clearly state what you want answered.
    – Mr. Shiny and New 安宇
    Commented Sep 10, 2013 at 15:18
  • @TomBurchell FWIW I think you are generally correct in your assumption that past tense must cannot indicate obligation, for the reason that I alluded to in my first (smart aleck) comment...you cannot obligate someone to do something in the past. However, having said that, if I ask my son "Must you have driven home?", that is interrogative in form but not necessarily in effect...it's more a suggestion that, in the future, he shouldn't drive home.
    – JeffSahol
    Commented Sep 10, 2013 at 16:28

1 Answer 1

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Must is a modal auxiliary verb, and like all such, has many restrictions and irregularities.
For one thing, must can't be used to refer to past deontic (obligation) senses. At all.
For another, must with a human subject and active predicate is usually interpreted as deontic.

  • He must be there tomorrow.
    (= he is obliged to be there tomorrow;
      ≠ he is almost certain to be there tomorrow)
  • *He must be there yesterday.

Must have is always epistemic (logical conclusion), rather than deontic.

  • He must have been there yesterday.
    (= he was almost certainly there yesterday;
      ≠ he was obliged to be there yesterday)

If you need to refer to a past obligation, you use the modal paraphrase have to;
it has a past tense, an infinitive, and a participle, where must doesn't,
so it can swing either way in the past or the present.

  • He has to be there tomorrow.
    (= he is almost certain to be there tomorrow;
      = he is obliged to be there tomorrow)

  • He had to be there yesterday.
    (= he was almost certainly there yesterday;
      = he was obliged to be there yesterday)

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    english.stackexchange.com/users/15299/john-lawler That answer seems fairly definitive, but can you give the source where and why the past deontic use can't be used. For example just in natural logical use of must "Must you have eaten all the food" seems correct in meaning and structure. I agree it is rare (as is any interrogative use of Must), but I really don't see a solid grammatical reason for it not to be correct.
    – Tom Burchell
    Commented Sep 10, 2013 at 3:00
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    Rarity has nothing to do with it. Must is already a past tense in form, but has no past meaning left. Must you have eaten all the food? may seem "correct in meaning and structure" to you, but it seems completely ungrammatical to me. Did you have to eat all the food? is what I'd say. Can you give me the source for your judgements on correctness in meaning and structure?
    – John Lawler
    Commented Sep 10, 2013 at 3:21
  • If so, what is this then? "To be eligible for this program, an applicant must have received a passing grade in all classes".
    – user1425
    Commented Oct 16, 2020 at 14:30
  • You ask about the name of the construction? Perfect infinitive (have received) complement of modal auxiliary (must). As for tense, modals have none.
    – John Lawler
    Commented Oct 16, 2020 at 15:16

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