Must reflects an obligation. It is usually used to talk about rules or prohibitions. For example:
- You mustn’t run by the pool.
- You must wear a helmet when you ride your bike.
- Pupils must raise their hand when they want a turn to speak.
- A receptionist must answer the phone when it rings.
- You mustn’t go swimming immediately after a meal.
Now it’s your turn, try to think of three things you must do, and three things you musn’t do.