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Pick just one:

  1. Tom can’t meet us because he must work.
  2. Tom can’t meet us because he has to work.

I don’t believe either sentence is wrong, although the grammar book prescribes has to as the answer.

Most grammar books say that have/has to is for external obligation, but what if Tom feels obligated to work? Can’t we then reasonably use must here?

Now, I believe has to is definitely the more common construction, but I don’t think must is entirely wrong.

Can someone shed some light on this?

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    In general, must is far less idiomatic in this context. It could be used in certain contexts. Suppose Tom is a portrait painter, you might say "Tom can't meet us today because he must get that portrait of Lady X finished by Saturday."
    – Kate Bunting
    Commented Sep 10, 2022 at 8:17
  • I take it then that “must” is possible, albeit rarely.
    – meepyer
    Commented Sep 10, 2022 at 8:24
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    All grammatical formations may be valid in some context or the other. In general, "he has to work [study / take rest / stay back]" is common & "he must work [study / take rest / stay back]" may be rare , but correct in some contexts where it is mandatory.
    – Prem
    Commented Sep 10, 2022 at 9:24
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    Note that sometimes we use "must" to disambiguate. For example: "This is a pair of glasses that she has to see." That sentence might mean that she has the glasses in order to see. However, the meaning is unambiguous if we write: "This is a pair of glasses that she must see."
    – MarcInManhattan
    Commented Sep 10, 2022 at 16:39
  • Many thanks everyone.
    – meepyer
    Commented Sep 11, 2022 at 1:44

1 Answer 1

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You're right - "must" is not entirely wrong here. It just isn't what native speakers of the English language would say in a casual conversation, such as the one in your example.

In relaxed settings, "has to" or "needs to" are far more common than "must," which has the same meaning but would occur more frequently in, say, Shakespeare. "Must" also occurs in formal settings (like speeches or legal arguments) and a few other special cases, which you'll have to pick up as you go along.

The external vs. internal distinction that you brought up is not a basis for distinguishing these terms.

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    I thought so. I teach level A2-B1 learners, and the rule in the grammar book about internal/external obligation really confused them. As a native speaker, I almost always prefer "have to" over must.
    – meepyer
    Commented Sep 11, 2022 at 1:45

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