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In English, to express strong obligation we can use either must or have (got) to. Grammars remind us that must is often used to express internal (personal) obligation, deduction (likelihood), and exhortation.

  1. The insulin shots for your pet must be given at twelve-hour intervals. (deontic)
  2. We must be late, there's no one in the foyer. (epistemic)
  3. If he wants to be healthier he must exercise. (deontic)

However, they tell us that have to tends to convey the rules and laws of an external authority which we have no choice but to follow and/or obey. The longer construction have got to is normally classified as being informal, and idiomatic in speech.

  1. I'm sorry but we have to leave early. (deontic)
  2. (a) You've got to believe me. (informal)
    (b) You gotta believe me. (very informal)

In my experience, this distinction between internal and external authority is very hazy and subjective, with the exception of sentence 2 where no obligation is expressed, native speakers use must and have (got) to more or less interchangeably.

In order to prove my point, consider how English expresses obligation in the past, the form had to is used whereas must is used with the perfect infinitive, i.e. must + have + past participle, to make speculations about the past and to convey certainty.

  1. The insulin shots had to be given at twelve-hour intervals.
  2. We must have been late, there was no one in the foyer
  3. If he wanted to be healthier he had to exercise
  4. We apologised and said we had to leave early.
  5. You had to believe me.

In the sentences with had to, the distinction between internal and external obligation is completely lost, greater context is required to know who the person or entity of authority is. In other words, HAD TO functions as the past for both HAVE TO and MUST. If this distinction, which many grammar sites (see below) explain is relevant, why does it disappear in the past?

Questions

  1. How and when did have to express the sense of obligation? What void did it fill?
  2. If deductions (epistemic) in the past can be expressed with must + have + PP what happened to deontic must in the past? Is there an etymological explanation?
  3. What happened to the distinction between internal (subjective) and external (objective) obligation when we speak about the past? Did it ever exist?

Sources:
Modals (1) Obligation
What's the difference between must and have to?
must / have to / have got to
Modals to express obligation: MUST, HAVE (GOT) TO
English modal verbs
Categorization principles of modal meaning categories

  • To have to for "must" (1570s) is from sense of "possess as a duty or thing to be done" (Old English). etymonline.com/word/have - Must - Used as present tense from c. 1300, from the custom of using past subjunctive as a moderate or polite form of the present. etymonline.com/word/must – user067531 May 16 '18 at 20:14
  • The relevant OED entry for have to: a. To be under an obligation to do something; to be required to; to need to. This usage developed from sense 8a: the duty or thing to be done was initially expressed as a direct object of the verb (to have something to do, sense 8a), then in an infinitive clause (to have to do something, this sense). Because word order was unfixed in early periods, it is difficult to determine precisely when this sense arose; the first unambiguous examples are those in which the verb in the infinitive clause is intransitive. (continued in the next comment) – linguisticturn May 16 '18 at 20:44
  • (continuing from the previous comment) Attestations such as quots. OE and a1225 are syntactically ambiguous, and may be transitional from sense 8a. It has also been suggested that in early use the construction may occasionally approach a periphrastic or modal future in sense rather than more narrowly implying obligation (compare the Latin constructions rendered in the quots.): (continued in the next comment) – linguisticturn May 16 '18 at 20:44
  • (continuing from the previous comment) OE West Saxon Gospels: Matt. (Corpus Cambr.) xx. 22 Mage gyt drincan þone calic ðe ic to drincenne hæbbe [L. bibiturus sum]. a1225 (▸c1200) Vices & Virtues (1888) 27 All ðat ȝe habbeð to donne [L. uultis facere], an godes name doþ hit, mit gode ȝeleaue. – linguisticturn May 16 '18 at 20:45
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(1) How and when did have to express the sense of obligation? What void did it fill?

Have to is an example of what's called a Periphrastic Modal (periphrastic is a technical term for 'paraphrased', meaning taking more than one word). Most English modal auxiliary verbs have at least one matching periphrastic modal construction, viz:

  • must ~ have to
  • should ~ ought to
  • can ~ (be) able to
  • may ~ (be) possible
  • will ~ (be) going to
  • will ~ (be) willing to

These constructions are common and have been around a long time. They arose because, as mentioned in another answer, English modal auxiliaries are defective verbs and therefore can't be used in many places where they could make sense because their morphology forbids it.

Thus, while it's possible to speak of a past obligation, you can't use must in the past to do so

  • *He musted go to the dentist yesterday.

but rather a periphrastic modal that does have a past tense

  • He had to go to the dentist yesterday.

Similarly for infinitives and participles

  • *I would hate to must rewrite my paper.
  • I would hate to have to rewrite my paper.
  • *He's musting rewrite his paper
  • He's having to rewrite his paper
  • *He has musted rewrite his paper
  • He has had to rewrite his paper

And similarly for the other periphrastic modals (examples left as an exercise).

(2) If deductions (epistemic) in the past can be expressed with must + have + PP what happened to deontic must in the past? Is there an etymological explanation?

Oh, yes. It turns out that must is itself based on an old preterite form, and there simply is no present form, which would likely be something like *muss if it existed in English.

German still has inflected modals, and the 3s present tense form of the modal verb müssen is er muss 'he must', while the past tense form is er musste 'he had to'. The final -t in German is the past tense suffix, and the final -t in English must used to be a past tense morpheme, but now it's just part of the word.

(3) What happened to the distinction between internal (subjective) and external (objective) obligation when we speak about the past? Did it ever exist?

Nothing happened to it. It's a zombie rule. Some people believe that it is real and that they always mean things that way, whatever they may actually say. But in fact it is not anything like general, as you note, and it doesn't seem to describe many uses of must and have to, let alone other modal-paraphrase pairs.

Executive Summary: Don't believe everything you read. Especially not about English grammar.

  • There was a present form in OE: mot. Any idea where this internal/external nonsense came from? My first thought was, “He has to go to the bathroom,” which is about as internal as you can get. – KarlG May 17 '18 at 0:49
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    Wishful thinking by English teachers who never tried to find counterexamples. There's a lot of that around. As for mot, I forgot so mote it be; sorry. – John Lawler May 17 '18 at 1:56
  • @KarlG I don't lke the rule, but I could see how someone could defend it by saying that in I have to go to the bathroom, the have to refers to an obligation that does not exist in my will, but outside of my will; it is the call of nature, not necessarily of me. I don't want to go to the bathroom at the 1:00 mark of the game but have to. – green_ideas May 17 '18 at 21:00
  • @user: In this age of language corpora, I expect broad claims like this "rule" to be supported by statistics. – KarlG May 17 '18 at 23:53
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"Must" is what's known as a "defective" verb, or one that list lacking particular conjugations. "Must" is the only form of the verb; it doesn't change for third versus first person, and it has no past tense form. And in fact, "must" doesn't really make sense in the past tense. Note that in the deductive sense, "must" is still in the present tense, even though it is speaking of past events. In "We must have been late, there was no one in the foyer", us having been late occurred in the past, but this having to be true is occurring in the present: it is presently true that us having been late occurred.

  • No verb in English except "be" changes from the first to second person. I also don't think English has conjugations, only verb forms. -s, -es, -ing, -ed. – Lambie May 16 '18 at 21:24
  • @LAmbie Almost every verb changes from first to second person. You presented examples in your very comment: "changes" is the second person version of "change". And the idea that English doesn't have conjugations is quite the novel one. – Acccumulation May 16 '18 at 21:48
  • I love tea; you love tea, s/he loves tea. Third person, not second person. English has verb forms. Have a look: as.wvu.edu/~pconner/Q&G3.html English is said to have conjugations because the grammarians were originally Latinists. Today, there is no need for it. – Lambie May 16 '18 at 22:16
  • All modal auxiliaries are defective verbs in English; they have no infinitive form, no present participle form, no past participle form, and no finite inflected forms. One form does all, no changes allowed. – John Lawler May 16 '18 at 22:46
  • @Lambie: English verbs are said to be conjugated because one doesn't wish to sever present day language from centuries of its history. If you read Shakespeare, you need to conjugate. – KarlG May 17 '18 at 0:56

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