The prominent authors of the book, "Exploring Grammar in Context: Grammar reference and practice upper-intermediate and advanced" says this on Must versus Have To on page 197:

Must and have to can occur together, for example in:

A: You must have to leave fairly early. (I suppose you are obliged to leave fairly early.)

B: Yes, I do

I am not a native speaker. But I am not comfortable with the use of these two verbs next to each other as they have the same meaning. It is redundant and outright wrong in my linguistic intuition of English.

Could this be a British thing? What do Americans think of it? Could this be only a spoken thing that is never found in careful academic writing?

Thank you for considering the question.

Magic79A: You must have to leave fairly early.

This is possible, but only when "must" expresses the speaker's guess/assumption about what is true, based on indirect evidence. It is not possible if "must" expresses obligation. Thus, your sentence means something like "I guess/assume that it is necessary for you to leave fairly early".


As a Canadian from Britain, You must have to leave fairly early is fine to me.


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Thanks Clive and GPY.

Now that you have reiterated the author's comment about "must have to", I completely understand why it makes sense.

Must is used for obligation and also for deduction.

It must be cold outside. Look at the snow! (Logical conclusion/deduction)

He must leave now. (Must expresses necessity/obligation)

He has to leave now. (Has to expresses necessity/obligation)

He must have to leave now. (This means that I am guessing that he has to leave now for one reason or another. So must is used here in its deduction sense, not in its obligation sense.)

Okay. I think I got it. Thanks.

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