Grammar (1ère) "must" and "mustn't"

A.Structures

• Simple present (affirmation) : S + must + infinitive without "to". Example : We must sleep early.

We can also use : have to /has to. Example : We have to sleep early. 

• Simple present (negation) : S + must + not /mustn't + infinitive without "to". Example : You mustn't wear dirty clothes.

  We can also use : don't have to / doesn't have to. Example : You don't have to shout on your parents.

  NB :

  For the past, use "Had to". Example : We had to sleep early. "Didn't have to". Example : You didn't have to drink alcohol.

  For the future, use "Will have to". Example : We will have to be on time. "Won't have to". Example : You won't have to drink alcohol.

B.Uses

• We use (must/have to/had to/will have to) to express obligation, give orders and give advice.

Examples :

We must go to the hospital when we are sick.

Children must go to bed early.

• We use (mustn't/didn't have to, won't have to) to talk about things that we need to avoid doing.

Examples :

Pupils mustn't eat in the classroom.

We mustn't pollute our environment.