Unit 28 — must and can't

1. 核心概念

这一单元里的 must 和 can't，不是在表达“必须 / 禁止”，而是在表达：

根据现有信息做判断和推测

核心区别：

结构	核心意思	判断强度
must	一定是 / 肯定是	很确定
can't	不可能是	很确定地否定

可以理解成：

must = 我几乎确定这是真的

can't = 我几乎确定这不是真的

---

A. must / can't：对现在情况进行判断

2. Example — It must be very noisy

My house is very near the motorway.

我家离高速公路很近。

听到这个信息以后，对方说：

It must be very noisy.

那里一定很吵。

这里的逻辑是：

房子离高速很近 ↓ 高速公路通常很吵 ↓ 所以我判断： It must be very noisy.

---

3. must = 我相信这件事一定是真的

图片中的规则：

We use must to say that we believe something is certain.

也就是：

当我们根据证据、常识或逻辑，认为某件事情几乎肯定是真的时，用 must。

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4. Example — You must be tired

You've been travelling all day.

你已经旅行了一整天。

所以：

You must be tired.

你一定累了。

推理过程：

travelling all day
        ↓
   travelling is tiring
        ↓
   must be tired

这里不是说：

你“必须”累。

而是：

根据情况判断，你肯定很累。

---

5. must 在这里不是“必须”

这是非常重要的一点。

must = obligation

You must wear a seat belt.

你必须系安全带。

这里：

must = 必须

---

must = deduction

You must be tired.

你一定很累。

这里：

must = 一定 / 肯定

所以要根据语境判断。

---

6. Example — You must be joking

“Joe is a hard worker.”

“Joe 工作很努力。”

“Joe? You must be joking. He doesn't do anything.”

“Joe？你一定是在开玩笑吧。他什么都不做。”

这里：

must be joking

表示：

根据我对 Joe 的了解，我判断你不可能是认真的。

所以：

你肯定是在开玩笑。

---

7. Example — Louise must get bored

Louise must get very bored in her job.

Louise 工作时一定经常觉得很无聊。

原因：

She does the same thing every day.

她每天都做一样的事情。

推理：

same thing every day
         ↓
     very repetitive
         ↓
  must get bored

---

8. can't = 我认为这件事不可能是真的

图片中的规则：

We use can't to say that we believe something is not possible.

也就是：

根据证据和逻辑，我认为某件事不可能成立。

---

9. Example — You can't be hungry

You've just had lunch.

你刚吃过午饭。

所以：

You can't be hungry already.

你不可能已经饿了。

这里不是：

“你不能饿。”

而是：

按常理判断，你现在不可能饿。

---

10. can't 的核心思维

推理过程：

just had lunch
      ↓
people don't usually get hungry immediately
      ↓
   can't be hungry

所以：

can't = logically impossible

---

11. Example — They can't know many people

They haven't lived here for very long.

他们住在这里没多久。

所以：

They can't know many people.

他们不可能认识很多人。

逻辑：

haven't lived here long
          ↓
  little time to meet people
          ↓
    can't know many people

---

12. must / can't 的基本结构

结构：

主语 + must / can't + 动词原形

常见形式：

结构	Example
must + be	You must be tired.
can't + be	You can't be hungry.
must + be + V-ing	You must be joking.
must + get	She must get bored.
can't + know	They can't know many people.
must + have	He must have a lot of money.

注意：

must / can't 后面直接接动词原形

---

13. must be

结构：

must be + 形容词 / 名词 / 地点等

例如：

He must be tired.

他一定很累。

She must be at work.

她一定在上班。

This must be the right place.

这里一定就是正确的地方。

---

14. can't be

结构：

can't be + 形容词 / 名词 / 地点等

例如：

He can't be serious.

他不可能是认真的。

She can't be at home.

她不可能在家。

That can't be true.

那不可能是真的。

---

15. must be doing

如果判断的是：

现在正在进行的动作

可以使用：

must be + V-ing

例如：

You must be joking.

你一定是在开玩笑。

时间重点：

NOW
 ↓
joking
 ↑
must be

---

16. can't be doing

同样：

can't be + V-ing

表示：

某人现在不可能正在做某事。

例如：

He can't be sleeping.

他现在不可能在睡觉。

---

17. must / can't 对现在的判断

判断	结构	Example
一定是某状态	must be	She must be tired.
不可能是某状态	can't be	She can't be tired.
一定正在做	must be doing	He must be working.
不可能正在做	can't be doing	He can't be working.
一定知道	must know	She must know him.
不可能知道	can't know	She can't know him.

---

B. must have done / can't have done：对过去进行推测

18. 核心概念

如果我们推测的是：

过去发生的事情

就需要：

must have + 过去分词

或者：

can't have + 过去分词

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19. Example — They must have gone out

Martin 和 Lucy 以为朋友在家。

他们：

rang the doorbell twice

按了两次门铃。

但是：

nobody has answered

没有人回应。

所以 Lucy 判断：

They must have gone out.

他们一定出去了。

也就是：

there is no other possibility

她认为：

几乎没有其他合理解释。

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20. 时间关系

现在看到：

nobody answers the door

然后回头推测：

之前发生了什么？

时间轴：

PAST                         NOW
  │                           │
  ● gone out ────────────────▶│
      ↑                        ↑
 must have gone          nobody answers

所以：

must have done = 根据现在证据推测过去一定发生过

---

21. must have done 的结构

must + have + 过去分词

例如：

must have gone

must have dropped

must have been

must have forgotten

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22. Example — I must have dropped it

I lost one of my gloves.

我丢了一只手套。

所以：

I must have dropped it somewhere.

我一定是把它掉在什么地方了。

这里：

现在：手套不见了 ↓ 推测过去：我肯定在某处掉了它

---

23. Example — It must have been noisy

We used to live very near the motorway.

我们以前住得离高速公路很近。

对方说：

It must have been noisy.

那里当时一定很吵。

注意时间：

现在已经不住在那里。

所以不能只说：

It must be noisy.

而是推测过去状态：

must have been noisy

---

24. must be vs must have been

现在	过去
It must be noisy.	It must have been noisy.
现在一定很吵	当时一定很吵
must + be	must + have been

时间轴：

must be

NOW
 ↓
noisy

must have been

PAST                    NOW
  ↓                      │
 noisy                    │
  ↑

must have been

---

25. can't have done

如果我们认为：

某件过去的事情不可能发生过

使用：

can't have + 过去分词

---

26. Example — She can't have got my message

Sarah hasn't contacted me.

Sarah 没联系我。

所以：

She can't have got my message.

她不可能收到我的消息。

推理：

if she had got the message
         ↓
   she would probably contact me

但：

she hasn't contacted me
         ↓
can't have got the message

---

27. Example — Max can't have been looking

Max walked into a wall.

Max 撞到了墙。

所以：

He can't have been looking where he was going.

他当时肯定没有看路。

这里的结构稍微更复杂：

can't have been + V-ing

表示：

过去当时不可能正在做某事

---

28. can't have been doing

结构：

can't + have + been + V-ing

例如：

He can't have been looking.

他当时不可能在看。

时间关系：

PAST                         NOW
  │                           │
looking?                      │
   ✕                          │

can't have been looking

---

29. 对过去推测的结构

推测	结构	Example
过去一定发生	must have + done	He must have left.
过去一定处于某状态	must have been	She must have been tired.
过去一定正在进行	must have been + V-ing	He must have been sleeping.
过去不可能发生	can't have + done	He can't have left.
过去不可能是某状态	can't have been	She can't have been tired.
过去不可能正在进行	can't have been + V-ing	He can't have been sleeping.

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30. 图片中的过去推测例句

Example	中文
They must have gone out.	他们一定出去了。
I must have dropped it somewhere.	我一定把它掉在什么地方了。
It must have been noisy.	当时一定很吵。
She can't have got my message.	她不可能收到我的消息。
He can't have been looking where he was going.	他当时肯定没在看路。

---

31. couldn't have 可以代替 can't have

图片中还说明：

You can use couldn't have instead of can't have.

也就是说，在过去推测中：

can't have done

和：

couldn't have done

经常都可以表示：

不可能发生过

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32. Example

Sarah couldn't have got my message.

Sarah 不可能收到我的消息。

等同于：

Sarah can't have got my message.

---

Max couldn't have been looking where he was going.

Max 当时不可能在看路。

等同于：

Max can't have been looking where he was going.

---

33. can't have vs couldn't have

结构	核心意思
can't have done	不可能做过
couldn't have done	不可能做过

在这里两者都可以表达：

对过去情况的强烈否定推测

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34. 现在推测 vs 过去推测

这是这一单元最重要的总结构。

时间	肯定推测	否定推测
现在	must + 动词原形	can't + 动词原形
过去	must have + 过去分词	can't have + 过去分词

例如：

现在	过去
He must be tired.	He must have been tired.
He can't be tired.	He can't have been tired.
He must be working.	He must have been working.
He can't be working.	He can't have been working.

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35. 一个非常重要的思维方式

不要机械背：

must = 必须

can't = 不能

在这一单元里更应该理解成：

must

根据证据，我认为只有这个解释最合理。

can't

根据证据，我认为这个解释几乎不可能。

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36. 推理强度

可以这样理解：

100% ────────────────────────────── 0%

      MUST                   CAN'T
 almost certain        almost impossible

must

接近：

100% 确定

但它本质上仍然是：

推测

不是亲眼确认。

---

can't

接近：

0% 可能

也就是：

我认为不可能。

---

37. must vs probably

例如：

He must be at home.

他一定在家。

比：

He's probably at home.

他大概在家。

确定程度更强。

可以粗略理解：

表达	确定程度
must	很高
probably	较高
may / might / could	有可能
can't	几乎不可能

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38. must / could / can't 的可能性梯度

可以这样建立一个简单模型：

VERY LIKELY
    │
    ▼
   MUST
    │
 probably
    │
may / might
  / could
    │
    ▼
  CAN'T
    │
IMPOSSIBLE

所以：

must = 一定

could / might / may = 可能

can't = 不可能

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39. 同一个场景对比

假设门是锁着的，Tom 没有回应。

你可能说：

很确定

Tom must be out.

Tom 一定出去了。

不确定

Tom might be out.

Tom 可能出去了。

认为不可能在家

Tom can't be at home.

Tom 不可能在家。

---

40. 过去版本

如果讨论的是昨天：

很确定

Tom must have gone out.

Tom 昨天一定出去了。

有可能

Tom might have gone out.

Tom 昨天可能出去了。

不可能

Tom can't have been at home.

Tom 昨天不可能在家。

---

41. 图片中的全部例句整理

现在推测

Example	核心理解
It must be very noisy.	一定很吵
You must be tired.	一定很累
You must be joking.	一定是在开玩笑
Louise must get bored.	她一定经常觉得无聊
You can't be hungry already.	你不可能已经饿了
They can't know many people.	他们不可能认识很多人

过去推测

Example	核心理解
They must have gone out.	一定已经出去了
I must have dropped it.	一定把它掉了
It must have been noisy.	当时一定很吵
She can't have got my message.	不可能收到消息
He can't have been looking.	当时不可能在看路
She couldn't have got my message.	不可能收到消息
He couldn't have been looking.	当时不可能在看路

---

42. 最重要的结构表

时间	状态	动作	进行中的动作
现在肯定	must be	must know/get/have	must be doing
现在否定	can't be	can't know/get/have	can't be doing
过去肯定	must have been	must have done	must have been doing
过去否定	can't have been	can't have done	can't have been doing

---

43. 时间轴理解

对现在进行判断

He must be tired.

PAST ───────────────────────▶ NOW
                               ↑
                          must be tired

重点：

我现在根据证据判断他的状态。

---

对过去进行判断

He must have been tired.

PAST                         NOW
  ↑                           │

must have been tired │

重点：

我现在回头推测过去的状态。

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44. 最实用的判断方法

先问两个问题。

第一步：我在判断现在还是过去？

如果是现在：

must / can't + 动词原形

如果是过去：

must / can't + have + 过去分词

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第二步：我是认为“一定”还是“不可能”？

如果：

一定

使用：

must

如果：

不可能

使用：

can't

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45. 四格判断法

	现在	过去
一定	must do	must have done
不可能	can't do	can't have done

这是这一单元最值得记住的表格。

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46. 母语思维

must

不是：

“我知道它是真的。”

而是：

“根据我现在掌握的信息，这是最合理的解释。”

例如：

The lights are on. He must be home.

灯亮着。

所以我推断：

他一定在家。

---

can't

不是：

“规则不允许。”

而是：

“根据逻辑，这个解释不成立。”

例如：

He left five minutes ago. He can't be home already.

他五分钟前才出发。

所以：

他现在不可能已经到家。

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47. 最终思维模型

现在

EVIDENCE
   │
   ▼
reasoning
   │

┌───┴────┐ ▼ ▼ MUST CAN'T 一定 不可能 │ │ ▼ ▼ NOW NOW

结构：

must / can't + do

---

过去

CURRENT EVIDENCE
       │
       ▼
    reasoning
       │
  ┌────┴────┐
  ▼         ▼
MUST       CAN'T
  │         │
  ▼         ▼
HAVE       HAVE
  │         │
  ▼         ▼
 DONE      DONE
      PAST

结构：

must / can't + have + done

---

一句话记忆

对现在推测

must do = 一定是 / 一定会

can't do = 不可能是 / 不可能会

对过去推测

must have done = 一定做过 / 一定发生过

can't have done = 不可能做过 / 不可能发生过

最核心的四格：

	NOW	PAST
一定	must do	must have done
不可能	can't do	can't have done

最终记住：

must = strongest positive deduction

can't = strongest negative deduction

have done = 把推测时间推回过去