Read these sentences and choose the modal verb 'must' or 'mustn’t'. 1. It's late. You
make so much noise.

2. You
tidy up your room. It is dirty.

3. They
do more exercises to stay fit.

1. It's late. You mustn't make so much noise.

In this sentence, we use 'mustn't' because it is giving a prohibition or a rule that you should not make noise because it is late.

2. You must tidy up your room. It is dirty.

In this sentence, we use 'must' because it is giving a strong recommendation or a strong obligation that you should tidy up your room because it is dirty.

3. They must do more exercises to stay fit.

In this sentence, we use 'must' because it is expressing a necessity or a strong recommendation that they should do more exercises in order to stay fit.