Form
Subject + mustn’t / can’t + Verb
You mustn’t smoke in here.
You can’t park here.
Meanings
Strong Prohibition & Rules
We use mustn’t (must not) to say that something is prohibited or against the rules. It is a very strong modal.
(A sign in a museum) Visitors mustn’t touch the exhibits.
(A parent to a child) You mustn’t talk to strangers.
Remember: Mustn’t is about what is not allowed. It is completely different from don’t have to (not necessary).