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I am teaching 'have to' vs 'must' (British English usage) and, according to the book, the difference is as follows:

must: it's necessary to do it (because the speaker says so)

have to: it's necessary to do it (because it's a rule or law)

The students had no difficulty with these ideas and applied them appropriately, both orally and in the initial exercises. But then we came across the following sentences:

  1. Passengers must / should remain in their seats while the plane is landing.

  2. Students must / may be silent during the written exam.

Both sentences present rules, not situations where the 'speaker says so', so why is the correct modal 'must' and not 'have to'?

I can only assume that the rules presented are incomplete (it happens the same when teaching Present Continuous, which is first taught without mentioning its future arrangement function). But I cannot find a single school book where any other rule, or even exceptions, are mentioned.

I have also searched EL&U and I did come across two related questions:

  1. "I have to" vs. "I must"

It's mentioned that 'have to' usually implies an obligation imposed by somebody other than the speaker and that 'must' is usually a personal obligation. This follows the line of the rule in my book and, again, fails to explain the sentences above.

  1. What's the difference between "I must help her" and " I have to help her"

It's mentioned that, although 'must' is supposedly for personal obligations it can be used for external obligations, and there's even the example "You must show your ID card" which is in line with the two examples from my book.

So I can't help but wonder: why is 'must' preferred in the two sentences above when the rules would have you use 'have to'? And also, when is 'must' preferred over 'have to' to express external obligation?

I'm particularly worried about this because the students became confused and I have no explanations for them.

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    I agree with the point made by Alex B in the first related question you linked in that "must" has a stronger feeling to it in face-to-face conversation than "have to" does. – John Clifford Feb 3 '16 at 18:59
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    must/have to help someone. For me, the must implies an over-arching sense of being urged to do something. Morally, for example. Whereas have to help her is just a duty. I don't like your original definitions: must implies a strong moral urge or agency to do something. Have to do something is less agency induced....Additionally, the Brits use must more in the sense of: Oh, it's late. I must go versus Ok, it's late, I have to go. Though AE also uses that at times in that way: strong urgency. – Lambie Feb 3 '16 at 19:32
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    must can mean obligatory as a result of a law in British English, for example in the Highway Code – Henry Feb 3 '16 at 22:01
  • @Henry The compulsion is not, necessarily, legal; it can also arise from natural forces and automatic processes. Think about the statement "What goes up must come down" (gravity is a natural force) or "It must stop raining soon" (meterological processes mean that there is a limit to the length of showers or longer periods of rain). – BoldBen May 17 '18 at 7:58
  • @BoldBen - it depends on the context. In the link I gave to a UK government website, it says "Many of the rules in the Highway Code are legal requirements. Such rules are identified by the use of the words 'MUST / MUST NOT'. " – Henry May 17 '18 at 8:30
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Depending on the context, the verb must can be used performatively. The verb have (to) cannot very easily with third person Subjects. What I mean by performatively, is that the actual uttering of the sentence is not a description of the necessity, but a directive, whereby the speaker is exerting their authority over the Subject just by uttering the sentence. The sentence causes the obligation, it doesn't merely describe it. If a speaker says:

  • Passengers have to fasten their seat-belts.

This is likely to be taken as a description of the fact that passengers must fasten their seat-belts.

However, if a speaker says (i.e. in this case writes):

  • Passengers must fasten their seat-belts.

This is likely to be taken as a directive whereby the reason that they have to fasten their seat-belts is because I'm telling them so in this utterance.

I disagree with the idea that have to is because of a third party saying so. The issue is that with must the speaker is yielding to the authority of some other obligator, or is obligating something themselves. Have to may represent some type of decree but the presentation is not one of obligation, but just necessity.

It can help to bring out this difference if you consider things that are necessary, but involve no morality, authority or obligation. For example:

  • You'll have to use this key to get into the house.
  • You must use this key to get into the house.

The first sentence seems to just be saying that this key is necessary for the purposes of entering the house, but there's no coercion, authority, morality or obligation involved. In the second sentence, the speaker seems to be saying something about some obligation to do things this way.

Understanding how modal verbs are used is very complex. I am not saying that any of these things are core meanings of these verbs. But they describe the implications that using these verbs often has.

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    Hi, Araucaria, just fixed one typo. I always understood the main difference between "must" and "have to" is the former could result in legal consequences unless taken seriously, e.g., "Passengers must fasten their seat belts (remain in their seat) while landing, otherwise, they would not be fully compensated for damages should there be an accident while landing." "Students must be silent, otherwise, they could get kicked out of the room or get failure on the exam." I agree must has to do with obligation. – user140086 Feb 4 '16 at 5:24
  • @Rathony Thanks for the edit. Am mulling over your interesting comment ... – Araucaria Feb 5 '16 at 13:27
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    I ain't never make non-interesting comment to you! :-) – user140086 Feb 5 '16 at 13:34
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'Must' is a modal auxiliary, and 'have to' contains an overloaded control verb and an infinitive marker (for the next verb). 'Have' is overloaded in that it can be a participle for the 'perfect' aspect, and it can mean possession, and (in this case) it can mean 'need'. Where this need comes from is not implied by either 'must' or 'have to', so they are synonymous. Any idiomatic nuance is not as clear as your book suggests. In terms of memory efficiency, modal 'must' is better than an extra control verb, but in terms of phonemes, 'hafta' may be easier to say.

  • Could I throw in a comment about negative forms as well, since Sara is going to need those as well. If someone says "You mustn't do something", it's a clear prohibition - you must NOT do it. If someone says "You don't have to do something", it's optional - OK to do it or not do it. – David Garner Feb 4 '16 at 11:05
  • But 'don't have to' is not the same as 'have to not'. I'll agree that there is no such option with modals. – AmI Feb 10 '16 at 17:14

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