15. We should/ ought to eat vegetable every day.
- It + be + advisable + for O + to V (Ai làm gì là phù hợp/ nên).
= S + should/ ought to + V-inf.
16. You mustn't smoke in this area.
- be (not) allowed to V: (không) được cho phép làm gì.
17. You may miss the test if you are usually absent from class.
- may = (will) probably (có thể, có lẽ).
18. It isn't necessary for you to tip in Vietnam.
- It + be + (not) + necessary + for O + to V-inf [Ai làm gì là (không) cần thiết].
= S + need(n't)/ not need to/ not have to + V-inf.
19. You have to go to school on time.
- be obliged to V: bị bắt buộc làm gì.