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Michael Vince in "English Grammar in Context" explains in his examples "You must be careful" is a speaker's personal opinion" while "You have to be careful" an outside opinion or rule" but with the first person there is little difference sometimes between "I have to go" and "I must go". So either can be used. But according to the book there is a difference between them when "have to" and "must" are used with "you".
Does Michael Vince mean that "must" with "you" means it is important for the speaker but with "have to" it is an outside rule?

In these examples either is used with "you". So I don't understand the purpose of the rule?

  • Doctor to patient: You must/have to take this medication three times a day.
  • Husband to wife: You must/have to take this medication three times a day.
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    Michael Vince must be one of very few people who can distinguish the difference in usage or significance between the two constructions.
    – Ronald Sole
    Commented Aug 21, 2021 at 16:18

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Husband to wife: You must/have to take this medication three times a day.

Without any additional context, "have to" (or "need to") sounds more idiomatic (in American English). "You must" could be interpreted as giving someone an order rather than explaining a rule/guideline established by somebody else.

But if the husband and wife are arguing about how often the medication should be taken, then "must" could be used for emphasis. ("You must take this medication three times a day or you could have a heart attack!")

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The difference is a nuance, and one that wouldn't always be noticed.

I do agree that there is a slightly different flavour between "you must" and "you have to" in the given context. I'm not sure that I can see much difference in the medicine example.

Even applying the rule suggests that "must" would be used when giving an opinion, but neither the doctor nor the wife is giving an opinion. So both should use "have to". In practice there would be a fair amount of variation.

I can easily imagine conversations with "must" that seem to express an external rule:

Son: Do I have to go to school.

Mum: Yes, you must.

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  • So even though the book says native speakers use "must" with "I" and "you" when talking about obligations that come from ourselves, but to talk about external rules with "I" and "you" native speakers use "have to" this isn't always true. Either is used when talking about personal obligations and external rules. With "you" also either "must" or "have to" is used to talk about both personal obligations and external rules. Am I right?
    – Antonia A
    Commented Aug 22, 2021 at 4:46

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