MUST and HAVE TO, strong obligation

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Modals, express a strong obligation or prohibition

« Must » and « Have to »: modals verbs, both express obligation.
« Have to » is a more flexible form.
Alternative: « have to », « should »
When do we use « must » or « have to »? Differences:

Must: modal auxiliary verb, followed by a main verb (infinitive without « to »)
Structure: subject + must + Infinitive verb without « to »
Examples: It’s late, I must go home.

MUST: We’re making a deduction, certain that something is true.
It must be nice to live in Nice: you have the sea and mountains very near.
You didn’t eat for 2 days? You must be hungry!
It’s impossible. That must be a joke.
alternative possible? « have to » (past, present, future).
That has got to be a joke (less common, but possible)

MUST: A strong obligation with internal circumstances.
The obligation is necessary by the person speaking. (personal, subjective).
The « obligation » is the opinion or idea of the person speaking.
You must stop smoking.
You must work harder.
You must come and visit us.
-> It’s not a real obligation. It’s not imposed from outside.

MUST: A necessity to do something.
American speakers prefer this form.
You must pass an exam to study in this university. (necessity idea).
alternative possible? « have to » (past, future).

MUST: Typical errors with the past and the future:
in the past: we use « had to ».
We don’t use MUST to expresses obligation / necessity in the past.
When my wife came home, she had to cook dinner for the children.
in the future: we use « will have to ».
We don’t use MUST to express obligation / necessity about the future.
I’ll have to speak to him quickly.

MUST: Criticisms
in criticisms, we use MUST with an interrogative form:
Must you keep doing noise? I can’t work!
Why must you forget my name? I’m Peter, not John!

HAVE TO: a necessary obligation with « have to », « have got to », « will have to »:
HAVE TO: A strong obligation with external circumstances.
I have to send a report to Head Office every week.
You have to pass your exams or the university won’t accept you.
I have to arrive at work at 8.30 sharp . I have a video conference with my boss every day!

HAVE TO: A strong obligation with « will have to ».
Some personal circumstance makes the obligation necessary,
It’s like « must ». (« will » is often used to show « willingness ».)
I’ll have to see with him.
I’ll have to get back to you on that.

HAVE TO = HAVE GOT TO (British English).
I’ve got to pay the ticket for the car, or I’ll get a fine.
You’ve got to let free the room before 12.00, or you will pay an other night.

A real obligation by a rule or a law?
MUST is ok, but generally we use have to for this.

Negative form: « must not » or « have not to » ?
Differences between « must » and « have to » are sometimes very small.
But there is a huge difference in the negative forms.
must not (or mustn’t): strong obligations NOT to do something.
must not (or mustn’t): used to prohibit actions. (very severe).
must not (or mustn’t): Only in negative form, Present & Future
(prohibition refers to the near future!).
You mustn’t talk about it. It’s confidential.
I mustn’t eat chocolate. It’s bad for me.
Kids, you mustn’t play in the street!
alternative possible? « should not », « ought to » (to dissuade rather than prohibit).

don’t have to: there is NO obligation or necessity.
I haven’t got to go. Only if I want to.
You don’t have to visit us (if you don’t want to).
We don’t have to do it (if we don’t want to).
They don’t have to buy it. (only if they want to).

REMEMBER: « Must not » vs. « Do not have to »
« Must not »: you are prohibited from doing something.
« Do not have to »: someone is not required to do something.
You must not do that. It is forbidden, it is not allowed.
You don’t have to do that. You can if you want to, but it is not necessary.

Tenses: For the present and the future, we use « must »:
I must go now. (present)
I must call my mother tomorrow. (future)
For the past, we use « have to »!