情态动词：建议、义务与可能 · PET / B1
天天向上 · 原创语法学习讲义
分清禁止与不必、建议与义务，再用 may / might / must / can’t 表达有依据的判断。
义务与必要
must、have to 表必须；过去的必要通常用 had to；将来的必要可用 will have to。
We had to change trains because the direct service was cancelled.
直达车取消了，我们不得不换乘。
must 与 have to 的实际用法有重叠，别把“内在/外在”当成绝对二分。
否定含义不同
mustn’t 是不能做；don’t have to / needn’t 是可以不做、没有必要。
You needn’t bring food, but you mustn’t bring glass bottles.
不必带食物，但不能带玻璃瓶。
先读规则是禁止还是免除义务。
建议与需要
should / ought to 提建议；need to 表需要，don’t need to 表不需要。
You ought to check the address before you leave.
出发前你应当核对地址。
ought to 后有 to，should 后没有 to。
推测的把握程度
may / might / could 表可能；must 可表达很有把握的推断；can’t 表有把握地排除。
This must be the right room: our names are on the door.
这一定是正确房间：门上写着我们的名字。
推断“不可能”常用 can’t，mustn’t 通常表达禁止。
语境练习
1. Notice: “You do not have to book in advance.” What does it mean?
A. Booking is optional.
B. Booking is compulsory.
C. Booking is forbidden.
2. School rules say visitors ___ smoke anywhere on the site.
A. do not have to
B. must not
C. need not
3. The lift broke yesterday, so we ___ use the stairs.
A. had to
B. must to
C. have to yesterday
4. You ___ wear more comfortable shoes for the walk.
A. should
B. should to
C. ought
5. That ___ be our guide; our guide is a woman and that person is a man.
A. doesn’t have to
B. can’t
C. mustn’t
6. “It might rain, so take a jacket.” Is rain certain?
A. No, it is a possibility.
B. Yes, it is guaranteed.
C. No, it is impossible.
7. The envelope has my full name and address. It ___ be for me.
A. mustn’t
B. must
C. needn’t
8. Which sentence means bringing lunch is unnecessary?
A. You need not bring lunch.
B. You must not bring lunch.
C. You cannot bring lunch.
答案与解释
1. A · Booking is optional.
don’t have to 表没有必要，不表示禁止。
2. B · must not
规则禁止吸烟，用 must not。
3. A · had to
过去不得不做某事用 had to。
4. A · should
should 后用动词原形；ought 必须有 to。
5. B · can’t
根据明确证据排除，用 can’t 表“不可能”。
6. A · No, it is a possibility.
might 表可能，不保证发生。
7. B · must
根据姓名地址作有把握的肯定推断，用 must。
8. A · You need not bring lunch.
need not / needn’t 表不必，而不是禁止。
表达任务
为学校出游写一段提醒：包括一条必须遵守的规则、一条禁止事项、一项不必做的事、一条建议和一种天气可能。
□ 禁止与不必没有混淆。
□ 建议、义务和推测的语气合适。
□ must / should 后原形，ought to / have to 结构完整。
参考表达
You must arrive at school before eight because the coach leaves at 8:15. You must not leave the group without telling a teacher. You do not have to bring lunch because the museum café will provide it. However, you should bring a bottle of water and wear comfortable shoes. It might rain in the afternoon, so a light jacket would be useful. Please ask if you need any help.
难度范围参照 Cambridge B1 Preliminary for Schools；本站原创练习，并非官方试卷。