情态动词不仅可以描述已经发生的事情,还能表达可能性、必要性、许可或意愿。它们数量不多,而且语法规则完全一致。
通用语法规则
所有情态动词的用法都相同:
- 第三人称单数不加 -s:She can swim.(不是「cans」)
- 后面接动词原形,不带 to:You should go.(不是「should to go」)
- 否定句和疑问句不需要 do:Can you help? / I can't come.
唯一的例外是 have to,它的行为像普通动词一样:She has to go. Does she have to go?
表达能力:can / could
- I can speak three languages.(现在的能力)
- I could swim when I was five.(过去的能力)
如果是指过去某一次具体的成功,需要用 was/were able to 或 managed to:
- ❌ The exam was hard but I could pass.
- ✅ The exam was hard but I was able to pass.
Could 描述的是一般性的能力;某一次具体做成了某事,需要用别的形式。
表达许可:can / may
- Can I borrow your pen?(日常、随意)
- May I borrow your pen?(正式且礼貌)
请求与提供帮助
| 功能 | 从不太正式到更正式 |
|---|---|
| 请求对方做某事 | Can you…? · Could you…? · Would you mind…? |
| 为自己请求许可 | Can I…? · Could I…? · May I…? |
| 主动提出帮助 | Shall I…? · Would you like me to…? |
表达建议:should / ought to
- You should see a doctor.
- You shouldn't work so late.
Should 是表达建议时最常用的日常词汇:友善,不带压力。Ought to 意思相同,但使用频率较低,语气略显正式。更强硬的建议可以说:You'd better see a doctor.(暗示不这样做会有不良后果)
表达义务:must 还是 have to
这是学习者最容易混淆的区别。
| 含义 | 例句 | |
|---|---|---|
| must | 说话人自己的权威或强烈的个人感受 | I must call my mother — I keep forgetting. |
| have to | 外部规则或客观情况 | I have to wear a uniform at work. |
在实际口语中,have to 出现的频率远高于 must。Must 多用于说话人自己制定的规则,以及书面告示:Passengers must not smoke.
还要注意,must 没有过去式和将来式——这两种时态都由 have to 承担:I had to work late. · I'll have to ask.
否定形式的陷阱
在否定形式中,这两个词的意思完全不同——这正是几乎所有学习者都会犯错的地方:
- You mustn't park here. → 这是禁止。
- You don't have to park here. → 这是没有必要(如果你愿意,还是可以停)。
Mustn't 表示禁止。Don't have to 表示没有义务。二者几乎是相反的意思。Needn't 与 don't have to 意思相近:You needn't wait.
表达可能性:might / may / could
- It might rain later.(可能,不确定)
- She could be at home.(可能)
在 B1 水平,这三个词表达的不确定程度大致相同。
表达推测的情态动词:你有多确定
同样的这些词还有第二种用法——表示你对当下正在发生的事情有多确定。
| 情态动词 | 确定程度 | 例句 |
|---|---|---|
| must | 几乎可以肯定是 | He must be tired — he's been up since five. |
| might / may / could | 可能 | She might be in a meeting. |
| can't | 几乎可以肯定不是 | That can't be right — I checked it twice. |
请注意:表示推测的 must 的反义词是 can't,而不是 mustn't。
对过去的推测也是同样的道理
加上 have + 过去分词,即可构成对过去的推测:
- He must have missed the train.
- She might have forgotten.
- You can't have seen him — he's abroad.
- I should have asked first.(对过去的遗憾)
同一个词,两种用法——must 要么表示义务(you must wear a helmet),要么表示推测(he must be exhausted)。区分二者的只有语境,所以要读懂情境,而不是只看这个词本身。
一段对话里的两种用法
A: Do I have to book in advance?
B: You don't have to, but you should — it can get busy.
A: It's Tuesday. It can't be that bad, surely?
B: You'd be surprised. They must have had a review somewhere.
Have to 和 should 表达义务与建议;can't 和 must have 表达推测。
自我检测
下面的测验重点考察最容易出错的区别——尤其是 mustn't 和 don't have to。