Talking about necessity and obligation

Must can be used in affirmative sentences to say that something is necessary. This is common in British English.

You must improve your English.
He must work hard.
I must be there before 9 o’clock.

In American English, have to is more common.

He has to work hard. She has to improve your communication skills.

Must can also be used to ask if something is necessary.

Must I finish the work today itself?
Must I clean all the rooms?

In American English, have to is preferred.

Do I have to finish the work today itself?
Do I have to clean all the rooms?

In British English, must not can be used to prohibit people from doing things.

You must not tell this to anybody.
You must not open this packet.

To say that things are unnecessary, we usually use ‘do not have to’ or ‘do not need to’.

You don’t need to be a rocket scientist to understand this.
If you have already applied, you don’t have to apply again.

To talk about past necessity, we can use had to.

When I was a child, I had to walk three miles to school.

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