Must (internal obligation) Have to (external obligation) (2025)

Hello teachers,
Is it true that the difference between "must" (internal obligation) and "have to" (external obligation) comes only from British English and not American English?
If so, in American English we use "must" and "have to" indistintevely to express internal or external obligation; right?

Thanks in advance.

My impression is that 'have to', generally, in AE, is related to 'external obligation'. But not always. Suppose a child blurts out something impolite about my wife's new dress. I might
express my displeasure: "You just had to say something, didn't you?"

Further, 'must' is quite often external, as in a sign in a parking lot: "You must get your ticket validated in the store where you do your shopping."

I can think of few generalizations or rules [about these words in AE] other than the one I've given above in blue.

bennymix said:

My impression is that 'have to', generally, in AE, is related to 'external obligation'. But not always. Suppose a child blurts out something impolite about my wife's new dress. I might
express my displeasure: "You just had to say something, didn't you?"

Further, 'must' is quite often external, as in a sign in a parking lot: "You must get your ticket validated in the store where you do your shopping."

I can think of few generalizations or rules [about these words in AE] other than the one I've given above in blue.

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Hello bennymix,
Thank you for your reply. Then, it is true that in American English the use of "must" and "have to" express internal or external obligation indistintevely.

TL

JordyBro said:

From my perspective
have to -- general requirements, sometimes things you feel you have to do based on a whim.
must -- logical assumptions and by extension requirements.

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Hello JodyBro,
Thank you for your reply.
Do you agree on these conclusions too?
I must stop smoking. (I want to stop smoking -- internal obligation)
The obligation can come from the speaker or the listener.

I have to stop smoking. (Doctor's orders -- external obligation)
The obligation comes from 'outside'. Never from the speaker.

TL

I don't know where you got this theory from, but all I can say is that you should treat it as a "tendency" at most, in American or British English. In most circumstances people may use either "must" or "have to" interchangeably to describe either an external or internal obligation.

In the smoking example, "have to" could also be used for an internal obligation.

Parla said:

They mean the same to me; no difference.

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Same for me.

Good points, Glasguensis. It's not hard to think of 'have to' for an 'internal compulsion'. A smoker is desperate to go out of the office for a smoke. He says to a co-worker. "It's been 4 hours. I have to have a cigarette!"

Glasguensis said:

I don't know where you got this theory from, but all I can say is that you should treat it as a "tendency" at most, in American or British English. In most circumstances people may use either "must" or "have to" interchangeably to describe either an external or internal obligation.

In the smoking example, "have to" could also be used for an internal obligation.

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Susan Y said:

Same for me.

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Hello Susan Y,
Thank you for your reply.
Aside:
< Second question needs its own thread. >

TL

Hello teachers,
I appreciate all your replies. I think trying to explain all the minor nuances would have a page for each word!

TL

Muhammad Khatab said:

When talking about internal obligation, we can use either "must" or "have to". Must is more formal. But when talking about external obligation, we only use "have to" and not "must". That's the solid rule concerning this part according to Cambridge English Grammar in Use for Intermediate Learners of English.

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As with all "rules" for English usage, and as noted above, this is good general guidance but not a hard-and-fast rule.

Visitors must remove shoes before entering the building.
All guests must check in at the registration desk.
Tax returns must be filed by April 15.

Florentia52 said:

As with all "rules" for English usage, and as noted above, this is good general guidance but not a hard-and-fast rule.

Visitors must remove shoes before entering the building.
All guests must check in at the registration desk.
Tax returns must be filed by April 15.

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These are all written notices. I can imagine these notices as persons making these obligations themselves. The notices here are the speakers. So "must" here is not about external obligations. It's the notice itself that makes the obligation.

Glasguensis said:

I strongly agree with Florentia52. These grammar guides are guides, not rules. Before you come into the house you must remove your shoes. Before you come into the house you have to remove your shoes. This is a verbal external obligation and as far as I’m concerned the two are entirely interchangeable.

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So are there situations in which "must" can't replace "have to", and vice versa?

Before you come into the house you must remove your shoes. (the speaker is my mother: internal obligation imposed by the speaker herself)
;Before you come into the house you have to remove your shoes. (the speaker is my sister: external obligation imposed by somebody else other than the speaker, in this case, the mother)

But in all, English native speakers use them interchangeably in this day and age.

Muhammad Khatab said:

But in all, English native speakers use them interchangeably in this day and age.

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I don’t use them interchangeably. I don’t think I ever use “must,” other than in a construction such as “It must have been 3 a.m. when I woke up” or “You must be exhausted.”

Florentia52 said:

I don’t use them interchangeably. I don’t think I ever use “must,” other than in a construction such as “It must have been 3 a.m. when I woke up” or “You must be exhausted.”

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I mean when it comes to obligation, natives use them interchangeably.