1
You mustn’t read the book if you don’t want to.
You don’t have to read the book if you don’t want to.
You don’t need to read the book if you don’t want to.
You needn’t read the book if you don’t want to.
The expressions mustn’t and don’t have to have very different meanings.
Mustn’t means that something is prohibited or not allowed: ‘You mustn’t use your phone during the exam.’
Use don’t have to, don’t need to or needn’t when something is optional or unnecessary: ‘You don’t have to come if you don’t want to.’
2
There is somebody at the door. Who must it be?
There is somebody at the door. Who can it be?
It mustn’t be her at the door. It’s too early.
It can’t be her at the door. It’s too early.
When we use modal verbs of deduction, we use must in affirmative sentences when we are sure that something is true: ‘The lights are on, so somebody must be at home.’
In questions, use can or could: ‘Who can it be?’ ‘Could it be Anna?’
For a negative deduction, use can’t, not mustn’t: ‘It can’t be Anna; she’s abroad.’
Must can be used in questions about rules or obligations: ‘Must I finish this today?’ However, we do not normally use it in questions asking what is probably true.