Is must disappearing?

As an obligation word in speech, largely yes, have to and have got to have taken over, and must can sound stiff or bossy. It remains fully alive for deduction (that must be it) and in written notices and instructions, so it is not going anywhere.

What about have got to?

It means the same as have to and is more informal: I’ve got to go. It is common in British speech, contracts to I’ve gotta in casual talk, and avoids the past entirely. You cannot say I had got to, so had to covers that.

Where does should fit?

It is weaker than both. Must and have to state an obligation; should gives advice you can decline. You should book early is a recommendation; you have to book early says the tickets will otherwise be gone.

Which one to useB1

Must vs have to

The short answer

Must is obligation you impose on yourself; have to is obligation imposed from outside. The difference matters far more in the negative: mustn’t means it is forbidden, while don’t have to means it is optional. Those are opposites, not variants.

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At a glance

Must or Have to?

Compare	MustYour own conviction. Internal.	Have toA rule, a law, someone else. External.
Where the obligation comes from	The speaker	Rules, circumstances, other people
Negative meaning	mustn’t = forbidden	don’t have to = not necessary
Past form	None: borrows had to	had to
Future form	None: borrows will have to	will have to
Questions	Rare and formal	Usual: do I have to?
Deduction	Common: she must be tired	Not used this way

How to choose

Ask these, in order

1
Is it a rule, a law or someone else’s requirement?
Have to.
You have to show ID at the door.
The venue’s rule, not the speaker’s.
2
Is it your own sense of what is necessary?
Must.
I must call my mother this weekend.
Nobody is making you.
3
Is the sentence negative?
Choose by meaning, because they are opposites. Forbidden takes mustn’t; unnecessary takes don’t have to.
You mustn’t tell her. / You don’t have to tell her.
Don’t do it. / It’s up to you.
4
Is it in the past or the future?
Have to, in both. Must has no past or future form of its own.
I had to work late, and I’ll have to again tomorrow.
Must simply cannot go there.

Side by side

Each one, in full

Must

When to use must

Your own conviction. Internal.

Must expresses obligation the speaker feels or imposes: I must stop doing this, you must see that film. In writing it also carries institutional authority: passengers must retain their tickets, which is why notices use it.

Its second job is deduction, and in modern English this is the commoner one: she must be exhausted, that must be the postman. Here it means “the only reasonable conclusion”, and the negative for that meaning is can’t, not mustn’t.

I must remember to book the tickets.

Self-imposed.

You must try this, it’s excellent.

A strong recommendation.

There’s no answer. They must have left.

Deduction about the past.

NBThe negative of this is they can’t have left, never mustn’t.

Have to

When to use have to

A rule, a law, someone else. External.

Have to reports an obligation that exists independently of the speaker: rules, laws, timetables, bosses, circumstances. It is the ordinary way to talk about obligation in speech, and far commoner than must.

It is also the only option outside the present, since must has no past, future, infinitive or participle. Had to, will have to, having to and to have to all fill gaps that must simply cannot.

Do I have to sign anything?

Asking about a rule.

She’s had to cancel three times.

Present perfect, must has no such form.

We might have to reschedule.

After another modal, only have to is possible.

Mistakes to avoid

What learners actually get wrong

You mustn’t come if you’re busy, meaning it’s optional.
You don’t have to come if you’re busy.
Mustn’t forbids. The sentence as written tells someone they are banned from coming, which is the opposite of the intended kindness. This is the single most consequential error in the pair.
I must work late yesterday.
I had to work late yesterday.
Must has no past form and borrows had to. The same applies to the future, will have to, and to any position after another modal.
You must to wear a helmet.
You must wear a helmet.
Must is a modal and takes a bare infinitive with no to. Have to keeps its to, which is where the confusion comes from, the two obligation forms behave differently in the same slot.
He mustn’t have heard me.
He can’t have heard me.
For deduction, the negative of must is can’t. Mustn’t always means prohibition, never “I deduce that not”, so this sentence says he was forbidden to hear you.

Now use it

Modal verbs

can, must, should, have to: obligation, advice and deduction. Reading a rule and being able to apply it are different skills, and only one of them shows up when you speak.

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Questions

Asked and answered

Is must disappearing?: As an obligation word in speech, largely yes, have to and have got to have taken over, and must can sound stiff or bossy. It remains fully alive for deduction (that must be it) and in written notices and instructions, so it is not going anywhere.
What about have got to?: It means the same as have to and is more informal: I’ve got to go. It is common in British speech, contracts to I’ve gotta in casual talk, and avoids the past entirely. You cannot say I had got to, so had to covers that.
Where does should fit?: It is weaker than both. Must and have to state an obligation; should gives advice you can decline. You should book early is a recommendation; you have to book early says the tickets will otherwise be gone.