现在时的推断情态动词（must、can't、might）

当你根据已有线索得出结论时，你在做的就是逻辑推断。这节课重点学习三个常用于推断现在情况的情态动词：must、can't 和 might。

She hasn't slept in two days. She must be exhausted.（几乎可以确定是真的）
He says he passed the test without studying. He can't be telling the truth.（几乎可以确定不是真的）
The lights are off — she might be asleep already.（有可能，但不确定）

这不是随意猜测，而是根据你所观察到的事实得出的合理结论。

核心句型： 主语 + must / can't / might + 动词原形 → It must be cold. She can't know. He might be home.

三个情态动词一览

情态动词	表达含义	例句
must	几乎可以肯定某事属实	She must know* — she's been working here for years.*
can't	几乎可以肯定某事不属实	That can't be* right — I checked the figures myself.*
might	只是有可能，并不确定	They might be* stuck in traffic.*

与所有情态动词一样，这三个词后面跟动词原形——情态动词后不加 to，不加 -s，不直接加 -ing。

Must：得出肯定结论

当线索明显指向某一方向，使你几乎可以确定某件事是真的，就用 must。

You've been on your feet all day. You must be tired.
She speaks six languages. She must enjoy learning languages.
He looks just like Tom. They must be related.

汉语常用"一定""不可能""可能"等副词来表达推断，动词本身不变；英语则要把 must / can't / might 放在主语后、动词原形前，后面的动词不能加 to、-s 或直接加 -ing。

注意不要把推断用的 must 和义务用的 must 混淆（例如"You must hand in your assignment on time"）。语境会提示区别：推断的 must 基于观察和证据，而非规则。关于义务表达，请参阅 Have to / must (obligation)。

Can't：得出否定结论

当线索使某件事几乎不可能成立时，就用 can't——它相当于 must 的否定推断形式。

The museum closes at 5, and it's already 6. It can't be open.
This can't be the right address — there's nothing here.
He can't know the answer — it was announced after he left.

注意： 否定推断用 can't，不用 mustn't。✅ It can't be true. — ❌ It mustn't be true. Mustn't 表示禁止（"不准"），不用于推断。否定推断的固定形式是 can't。

Might：不确定的可能性

当你真的不确定，线索不足以作出明确判断时，用 might（或 could）。

She's not answering. She might be in a meeting.
It might mean something different in that context — I'm not sure.
He might know the answer — it's worth asking.

Might 比 must 的语气弱得多。说 She must be in a meeting 表示相当有把握，而 might 则表示只是一种可能。

关于用 may、might 和 could 表达可能性（而非推断），请参阅 Modals of possibility (may, might, could)。

对正在发生的动作作推断

当你想推断某个动作此刻正在进行，使用 must / can't / might + be + 动词的 -ing 形式：

The music is so loud — they must be having a party.
He's only wearing a T-shirt. He can't be feeling very warm.
She's not at her desk. She might be having lunch.

推断	句型	例句
几乎确定正在发生	must be + -ing	They must be waiting* for us.*
几乎确定不在进行	can't be + -ing	She can't be sleeping* — the lights are on.*
可能正在发生	might be + -ing	He might be working* late — no one can reach him.*

确定性层级：推断与可能

确定程度	情态动词	例句
几乎确定（肯定）	must	She must know* — she's the expert.*
不确定，开放式	might / could	She might know* — it's worth asking.*
几乎确定（否定）	can't	She can't know* — she just arrived.*

Must 和 can't 都表示高度确定——must 用于肯定推断，can't 用于否定推断。Might 居中，用于线索不足以作出判断的情况。

常见错误

❌ It mustn't be true — the numbers don't add up. → ✅ It can't be true.（mustn't = 禁止；否定推断用 can't）
❌ She must knows the answer. → ✅ She must know the answer.（情态动词后接动词原形，不加 -s）
❌ He must be know the answer. → ✅ He must know the answer.（原形动词前不加多余的 be）
❌ They must work in there right now. → ✅ They must be working in there right now.（正在进行的动作：must be + -ing）
❌ She can't to be serious. → ✅ She can't be serious.（情态动词后不加 to）

检验一下

为每句推断选择最合适的情态动词。

The exam was three hours long, she finished in forty minutes, and she got every question right. She ____ be very good at maths.（must / can't / might）
I can hear voices inside. Someone ____ be home.（must / can't）
His cash is always in his wallet, and his wallet is at home. He ____ be carrying any cash.（must / can't）
She's wearing headphones, but the screen is turned away from us. She ____ be listening to music.（must / might）

显示答案

She must be very good at maths.（速度和准确性是明确的线索）
Someone must be home.（听到声音是直接证据）
He can't be carrying any cash.（钱在钱包里，钱包不在身上——几乎不可能）
She might be listening to music.（有可能，但我们看不到屏幕，无法确认）

要点总结

Must = 几乎确定是真的——线索有力地支持这一结论。
Can't = 几乎确定不是真的——结论与你所知的事实相矛盾。
Might（或 could）= 有可能但不确定——线索不足以作出判断。
三者后面都接动词原形：不加 to，不加 -s，不直接在情态动词后加 -ing。
推断正在进行的动作：加 be + -ing → must be working，can't be sleeping，might be waiting。
Can't 是否定推断的形式，不用 mustn't（mustn't 表示禁止）。
对过去的推断有不同句型：must have / can't have + 过去分词——这是 Modals of deduction: past 的内容。