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.