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I have seen uses of must that appear to be in the simple past tense. Sometimes these seem grammatical, but sometimes not. Examples that help illustrate my confusion:

He knew he must go to New York - sounds fine to me.

He went to New York because he must - sounds questionable.

Because he must go to New York, he bought plane tickets - sounds completely ungrammatical.

He must go to New York - grammatical, but with no other verbs to provide context, can't interpret this as being past-tense.

My question: is this past-tense use of must ever acceptable? Is it only acceptable in certain contexts - if so, what are those contexts and why?

(Note - I'm not looking for had to. Have is a different verb, so had to expresses the same meaning as the past tense of must, but it itself is not the past tense of must.)

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The only time I've heard must used in a past tense is to say that one "must have" done something, which isn't quite the same construct. I would certainly say that for common English the past is either "had to" or it doesn't exist –  Ben Brocka Dec 8 '11 at 0:55
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Etymologically, "must" is the past tense of "mote" (as in "so mote it be"), so one wouldn't expect it to have a past tense itself. But just like the other "past tense" auxiliary verbs "could", "should", "would", "might", its meaning has become divorced from the fact that it used to be a past tense. –  Peter Shor Dec 8 '11 at 0:58
    
Like "had", you can always find a workaround using a verb that is marked for past tense: "He needed to go to New York" or "Because he [felt/knew] he must go to New York, he went". –  ShreevatsaR Dec 8 '11 at 1:44
    
@PeterShor: +1 This should have been an answer, rather than a comment, I think. –  Kris Dec 8 '11 at 6:08
    
@alcas: +1. Could you consider adding if such a past tense form exists in other languages? The question seems otherwise redundant or at least limited in scope. –  Kris Dec 8 '11 at 6:13
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7 Answers

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Rarely, must is used as a past tense. Belshazzar, by H. Rider Haggard, has we went because we must, in a prose style which is perhaps deliberately archaic to reflect the ancient Egyptian context.

In this odd snippet, If Thoreau went because he would, Hawthorne went because he must, one might say the author is "playing with language".

But here's Ralph Waldo Emerson with What he did, he did because he must. I would not wish to say Emerson doesn't know his own language.

From comments under @Henry's answer, it seems something quite odd has been going on. Many people will know the archaic present tense mote because Freemasons & such still say So mote it be in a "ritual" context. Bizarrely, the past tense "must" eclipsed "mote" for present tense usage. But in so doing, "must" somehow almost completely lost its ability to still be used as a past tense.

In spite of all the above, ordinary mortals in ordinary contexts today should stick with the standard position put forward by other answers. Use had to for the past tense.

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+1 for an illustrative answer. –  Kris Dec 8 '11 at 6:11
    
Thanks! Was not aware of "mote". It's interesting that (according to that link) "So mote it be" means "May it be so", not "It must be so". –  alcas Dec 8 '11 at 21:51
    
@alcas: I think when you try to "translate" words like that, you risk finding a difference that isn't really there. IMHO, So mote it be can be just as accurately converted to So must it be. By bringing in "may", you're introducing a note of supplication to a higher authority (God), that's not really implied by "mote". –  FumbleFingers Dec 8 '11 at 21:59
    
@FumbleFingers I think there's a more specific denotational difference: "So must it be" means "It has to be like this", whereas "So may it be" means "Let it be like this." The former expresses certainty/obligation; the latter is a hope/command. –  alcas Dec 8 '11 at 22:28
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Note that it's only possible in a past sense when it's final in the clause. I.e, We went because we must is OK, but *We went because we must go, unless it means We always must go, generically, and not We were obliged to go, we had to go. Must by itself you can get away with, but add anything to draw attention to it, and you get ungrammaticality. –  John Lawler Sep 10 '13 at 3:26
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There's a related discussion of modals, including must, here.

Though most of the information the questioner requested has already been provided here, it seems.

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All modal verbs, of which must is one, are invariable. That is to say, they do not inflect for person, number or tense. There is no *musted. If we wish to express the past sense of a modal verb, we have to do so by other means. In the case of must we use, as you have said, had to.

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Some modal verbs seem to be past forms of others, such as shall/should, will/would, can/could and perhaps may/might –  Henry Dec 8 '11 at 16:02
    
@Henry: Yes, there are obviously difficulties in trying to summarise something like modal verbs in a couple of sentences. We might, for example, say ‘I can play the piano now, but I couldn’t when I was younger.’ That, however, is not the usual meaning of ‘could’. It is more often used to express ‘unreal’ meaning, to seek permission and to make a suggestion. –  Barrie England Dec 8 '11 at 16:34
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The Chicago Manual of Style devotes Section 147 of its "Grammar and Usage" chapter to must:

Must denotes a necessity that arises from someone’s will [we must obey the rules] or from circumstances [you must ask what the next step is] [he went to New York because he must].

Must also connotes a logical conclusion [that must be the right answer] [that must be the house we’re looking for] [it must have been Donna who phoned].

This verb does not vary its form in either the present or past indicative. It does not have an infinitive form (to have to is substituted) or a present or past participle.

Denoting obligation, necessity, or inference, must is always used with an express or implied infinitive [we must finish this design] [everyone must eat] [the movie must be over by now].

But, to answer the question, I've found this guideline useful (it's not where I originally heard it, but its formulation is the same):

Use had to in past tense independent clauses, and use must in dependent clauses (usually noun or adjective clauses) when the sense is that of strong obligation or strong probability.

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To express the past tense of must another verb is used, except when it used in the reported speech.

  • reported speech

    She said she must be going.

  • certainty

    That must have been the right restaurant. There are no other restaurants on this street.

  • necessity

    We had to have a permit to enter the park.

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The reason that must does not seem to have a past tense in English is that etymologically it already was a past (or preterite) tense of motan.

Compare with the modern Dutch where ik moet means I have to while ik moest means I had to.

If you will not use had to, you can often use needed to, which fits your examples, or something similar.

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motan and mote are the same verb, separated by several centuries. –  Peter Shor Dec 8 '11 at 1:44
    
@Peter: There does seem to be something odd about the idea that present tense "mote" disappears in favour of what by that logic was the past tense of the same word. From which I infer that during the transitional period at least, "must" must have covered both tenses, in which case there's no fundamental reason why it can't continue to do so. –  FumbleFingers Dec 8 '11 at 2:01
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@Fumble: motan was the infinitive, mot the 1st and 3rd person singular present indicative, mote the singular present subjunctive and imperative. –  Henry Dec 8 '11 at 2:52
    
I guess you guys are right about "must" being somehow inherently past tense when we adopted the word. It still strikes me as weird in that it must have subsequently displaced "mote" for present tense, and later still, lost most if not all of its ability to be used as a past tense. –  FumbleFingers Dec 8 '11 at 15:42
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You can express it like:

He went to New York because he had to.

He will have to go to NY.

and such on.

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Did you read the part where I said I wasn't looking for had to? –  alcas Dec 8 '11 at 0:59
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