Expressing Obligation , Prohibition and absence of Obligation

To express obligation we use : must and have to .

1/ Expressing Obligation :

we use must and have to to express obligation 

Must or have to + verb in the infinitive

For example: 

You have to stop smoking .

You must have a passport to visit Italy.

I must study hard to pass my BAC exam.

2/ Expressing Prohibition :

We use must not to express prohibition.

Must not + verb in the infinitive

For example :

You must not use the mobile phone in the classroom.

Children must not enter this area ,they may harm themselves.

3/ Expressing Absence of Obligation :

We use do not have to , need not and do not need to , to

express absence of obligation.

Do not have to , need not , do not need to + verb in the 

infinitive

For example :

You do not need to play football.

I do not have to wear the uniform outside school.