Must.
by Danilo Rivera
1. You must not eat that. It is forbidden, it is not allowed.
2. You don't have to eat that. You can if you want to, but it is not necessary.
3. You must not swim in that river. It's full of crocodiles.
4. "Must" is most commonly used to express certainty. It can also be used to express necessity or strong recommendation
5. Jenny, you must not play in the street.
6. negative form.You mustn't drink so much. It's not good for your health.