Here are some useful ways to talk about general obligations.
You can use "must" to talk about what's obligatory or compulsory.
Remember: must is a modal verb, and is followed by the infinitive without "to":
"You must leave by 8pm."
You don't need to add 's' for third person singular:
"He must leave by 8pm."
The negative form is mustn't, and it means "not allowed":
"You mustn't smoke here." (i.e. it's forbidden.)
Using have to
This means much the same as "must", although we tend to use it more for general rules and regulations.
"You have to drive on the left in the UK."
It's not a modal, which means that the ending changes for third person singular:
"He has to start work at 6am every day."
For the negative or question form, use do / does:
"Do you have to leave early today?"
"Does she have to answer the phone at work?"
"I don't have to wear a uniform at work."
"She doesn't have to work weekends."
A stronger form of "have to" is "have got to":
"I've got to leave early today."
"He's got to stay late."
(For the negative and question forms, use the auxiliary "have")
"He hasn't got to leave early."
"Have you got to work shifts tomorrow?"
Remember: don't have to / haven't got to means "it's not necessary"
Using need to
We can also use "need to" to mean "have to":
"You need to work at least 35 years before you can get a pension."
(For the negative and question forms, use the auxiliary "do")
"Do you need to work nights?"
"I don't need to make the tea at work."
You can also use "needn't" (a modal auxiliary) for the negative:
"You needn't work nights when you've been here more than a year."
(However, this is quite old-fashioned and not as common as "don't need to".)