mustn't 和 needn't 有什么区别？情态动词否定回答的高频考点
引言
情态动词里最容易考、也最容易错的一组：mustn't 和 needn't。很多同学以为它们意思差不多，其实一个是"禁止"，一个是"不必"，完全相反。
这篇文章把情态动词否定回答的高频考点讲清楚。
mustn't vs needn't
mustn't = 禁止（绝对不可以）
```text
You mustn't play football on the road.
（你绝对不能在路上踢足球。）
Students mustn't use phones in class.
（学生上课禁止用手机。）
```
needn't = 不必（不需要）
```text
You needn't come tomorrow.
（你明天不必来了。）
We needn't hurry. There's enough time.
（我们不必着急，时间够。）
```
对比：
```text
You mustn't tell him. 你绝对不能告诉他。（禁止）
You needn't tell him. 你不必告诉他。（不强制）
```
Must I...? 的回答
```text
Must I finish it now?（我必须现在完成吗？）
肯定回答：Yes, you must.（是的，必须。）
否定回答：No, you needn't.（不，不必。）
```
注意：否定回答不能说 No, you mustn't.（那是"禁止"的意思，不是"不必"）
must vs have to
must —— 说话人主观认为"必须"
```text
I must study hard.（我必须努力学习。——我自己觉得）
```
have to —— 客观情况"不得不"
```text
I have to wear a uniform at school.
（我在学校不得不穿校服。——学校规定）
```
否定形式
```text
mustn't = 禁止
don't have to = 不必（= needn't）
```
```text
You mustn't tell anyone. 你禁止告诉任何人。
You don't have to tell anyone. 你不必告诉任何人。
```
can't 的两个意思
一、不能 / 不会（表能力）
```text
I can't swim.（我不会游泳。）
```
二、不可能（表推测）
```text
He can't be at home. The light is off.
（他不可能在家，灯没亮。）
```
一张表总结
```text
表达 意思 例句
mustn't 禁止 You mustn't smoke here.
needn't 不必 You needn't worry.
don't have to 不必 You don't have to come early.
must 必须（主观） I must remember.
have to 不得不（客观） I have to go now.
can't 不能/不可能 He can't be right.
```
总结
情态动词否定回答的核心：
```text
Must I...? → Yes, you must. / No, you needn't.
mustn't = 禁止（不许做）
needn't = 不必（不用做）
```
```text
今日小抄：
mustn't = 禁止 You mustn't park here.
needn't = 不必 You needn't pay.
Must I go? Must I go?
Yes, you must. Yes, you must.
No, you needn't. No, you needn't.
```