There are two modal verbs to show obligation or necessity.

Must
Have to

Must

Must is for simple present and usually shows general
obligations.

Travellers must show their passport before boarding a
plane.
You must use a pencil on the test.

It is formal and normally used only in writing.

All employees must wear proper safety equipment.

Have to

Have to is not an actual modal verb, but it is used like a modal.

You must conjugate the verb "have" depending on the time
and subject.

I have to work tomorrow.
She has to work tomorrow.

It is used in all tenses.

I had to work last night.
She has had to work every day this week.

"Have to" is less formal than Must and is common in
conversation.

Must not and Don't have to

"Must not" and "don't have to" are completely different, and
their meanings are often confused.

"Must not" is a negative obligation and shows something that
is prohibited.

Example: