must / mustn't

We use must + base verb to say that something is necessary or obligatory — it is a rule or a strong duty that you cannot ignore. Must has the same form for every subject: I must, you must, he must, she must, we must, they must. There is no -s in the third person singular. We use mustn't (must not) to say that something is forbidden — it is not allowed. This is different from 'don't have to', which means it is not necessary but is still possible; mustn't means it is NOT permitted at all. The negative mustn't is a strong prohibition: if you mustn't do something, doing it is against the rules or the law. To make a question with must, we put must before the subject: 'Must I wear a uniform?' For example: At school, you must arrive on time and you mustn't use your phone during the lesson.