• You must realise that school is not a prison.
• You must remember to take the test tomorrow.
• You must show your passport at the gate.

You can use have to, have got to and must to express the idea that something is necessary or required. (obligation)

Have to is used for all tenses and forms

• You have to be here at 10 o'clock.
• She had to say the truth to the judge.

Have got to and must are only use for the present.

 BE CAREFUL !  Don't have to and mustn't have very different meanings.

Don't / doesn't have to expresses that something is not necessary.

• You don't have to do the dishes.

Mustn't expresses prohibition. It means that something is not allowed or is against the law. There is no choice.

• You mustn't cross the road when the traffic lights are red.