must vs have to — speaker-duty vs outside-duty, and must's two walls (GiU Unit 31)

The split

	must	have to
source	speaker feels the necessity	external — rules, circumstances
examples	I must hurry (my feeling) · You must see that film (my urging)	I have to wear a uniform (company rule) · She has to work nights (circumstance)

Not interchangeable when reporting someone else’s external obligation — the exam forces Tom; the speaker isn’t feeling/urging it:

Tom isn’t going out this evening. He has to study for his exam. ✓ (KEY) ✗ He must study — must = the speaker’s own felt necessity, or direct urging of the listener. Reporting a third person’s outside-forced duty takes have to, even one-time.

must’s two walls

wall	fix	example
no past	had to	I couldn’t get a taxi last night. I had to walk home. (✗ must)
no routines/habits	have to	I have to travel a lot for my job. (✗ must)

must’s valid ground: present/future self-duty (I must remember to call Sarah), specific necessity (he must study tonight), warm insistence (You must come and see us again!).

Past deduction is a different must: He must have walked home = almost certain he did (Unit 28 deduction, not obligation).

The mustn’t trap

• mustn’t = it is forbidden — You mustn’t tell anyone.
• don’t have to = it is not necessary — You don’t have to come if you’re busy.

Negatives point opposite ways. GiU Unit 32 drills it: Don’t drop that vase → mustn’t; needn’t/don’t-have-to = optional.

Test anchors

It was good to see you. You must come and see us again soon. ✓ (insistence — direct urging) I must remember to call Sarah tomorrow. ✓ (self-duty) I couldn’t get a taxi last night. I had to walk. ✓ (past) I have to travel a lot for work. ✓ (routine) Tom isn’t going out. He has to study for his exam. ✓ (KEY — reported third-person external duty; ✗ must)

Source

Raymond Murphy, English Grammar in Use, Unit 31 — have to and must. Ebot lesson 2026-09.