|must||do your homework.|
|mustn't||touch the pictures.|
The modal verb must (negative: mustn't) is used when you think it is necessary to do something. Must expresses strong obligation.
Must / Mustn't have the same form in all persons.
Must is followed by the infinitive without to.
- It's a fantastic film. You must see it.
- The windows are very dirty. I must clean them.
- I mustn't forget to phone Tom.