Ответы
1. You mustn't be late for school.
2. You must go to the dentist with a toothache.
3. A teacher must be a patient person.
4. You musn't speak loudly in a library.
5. It's raining. You must take your umbrella.
6. You must stop when you see that sign.
7. You mustn't eat in class.
8. You must keep off the grass!
9. You must take a cough syrup once a day.
10. You mustn't tell my secret to anybody.
11. You mustn't watch too much television.
12. You must feed your pet twice a day.
13. You mustn't run with a broken leg. (Well, as a matter of fact, you can't, so...)
14. We must protect endangered species.
15. You must have a shower every day.
16. You must fasten your seatbelt in a car.
17. You mustn't forget about Mother's Day.
18. You must put the rubbish in a bin.
19. You must eat more fruit.
20. You mustn't listen to loud music after 10 p.m.
21. You mustn't eat junk food too often.
22. You must stop smoking. It's bad for you.
23. Students mustn't forget their homework.
24. You must keep the park clean.
25. You have a sore throat. You mustn't eat ice cream.