情态动词题数量不多，但 mustn't 和 needn't 一道就能拉开差距：Must I finish it today? 的否定回答是 No, you needn't，答成 mustn't 直接判错。规则不多，重点是把语气分层记牢。
常用情态动词对照表
| 情态动词 | 含义 | 例句 |
|---|---|---|
| can / could | 能力；许可；可能 | She can swim. Can I use your pen? |
| may / might | 许可；可能 | May I come in? |
| must | 必须（主观义务） | You must finish it before nine. |
| have to | 不得不（客观要求） | I have to look after my sister today. |
| should | 应该（建议） | You should drink more water. |
| needn't | 不必 | You needn't worry about it. |
两条铁律与常见搭配
- 铁律一：情态动词后面一律接动词原形，不加 s、不加 to——He can swim 不说 can swims
- 铁律二：情态动词的疑问句把它提到句首，否定句直接在其后加 not
- must 的否定是 mustn't，但意思变成「禁止」：You mustn't play with fire.
- 「不必」要用 needn't 或 don't have to 表达
- may 开头的问句，肯定回答用 Yes, you may / can，否定用 No, you mustn't
易错点：must 开头问句的否定回答
Must I hand in the homework now? 回答 Yes, you must；否定回答必须用 No, you needn't（或 don't have to），绝对不能说 No, you mustn't——mustn't 的意思是「不许、禁止」，答出去意思完全相反。考题专设这个坑，判断方法先想清楚两句话的中文意思：问句问「必须吗」，回答「不必」用 needn't，回答「禁止」才轮到 mustn't。