一、核心特征（语法重点）
动词原形：情态动词后接动词原形（不带 to）。
✅ He can swim.
❌ He can to swim / He can swimming.
无人称和数的变化：主语是单数第三人称时，情态动词本身不变。
✅ She must go. (不是 musts)
时态形式有限：大多数情态动词没有不定式、分词形式，也没有过去分词（除了 have to）。
否定句：直接在情态动词后加 not。
✅ You cannot (can't) come in.
疑问句：将情态动词提前至句首。
✅ Can you help me?
二、主要情态动词用法对比
情态动词
核心含义
典型用法与示例
Can / Could
能力
请求/许可
可能性
• 能力：Babies can't talk. (能力)
• 请求 (Could 更礼貌)：Could you pass the salt?
• 可能性：It can be very cold here. (有时会)
May / Might
许可
可能性
• 许可 (正式)：May I come in?
• 可能性：Take an umbrella, it might rain. (Might 比 May 可能性更小)
Will / Would
意愿
请求/建议
习惯
• 意愿：I will help you. (愿意帮忙)
• 请求 (Would 更礼貌)：Would you like some coffee?
• 过去习惯：He would read aloud every morning.
Shall / Should
建议
责任/预期
• 建议 (常用于第一人称)：Shall we go?
• 责任 (应该)：You should study harder.
• 预期：He should be here by now. (按理说应该)
Must / Have to
必须
肯定推测
• 必须 (主观/客观)：I must go now. / You have to wear a seatbelt.
• 推测 (肯定)：You must be tired. (你肯定累了)
Ought to
责任/建议
• 应该：You ought to listen to your parents. (语气比 should 强)
三、四大核心用法详解
1. 表“能力”：Can vs. Could vs. Be able to
Can：现在的能力。
I can speak English.
Could：过去的一般能力。
When I was young, I could run fast.
Was/Were able to：表示在过去某一次特定场合成功地做了某事（相当于 managed to do）。
The fire spread quickly, but everyone was able to escape. (强调成功逃脱)
2. 表“许可”：Can vs. May vs. Could
Can：口语中最常用。
May：非常正式、礼貌。
Could：比 Can 更委婉、客气。
注意：在回答别人的请求时，避免使用 Could/Might（显得不自信），应用 Can/May。
A: Could I use your phone?
B: Yes, of course you can. (不能说 Yes, you could)
3. 表“必须”：Must vs. Have to
Must：强调说话人的主观看法（“我认为必须”）。
Have to：强调客观需要或规则（“外界规定必须”）。
I must study hard to pass the exam. (我主观想通过考试)
You have to wear a uniform at school. (学校规定)
否定意义大不同：
Mustn't：表示禁止（“不准”）。You mustn't smoke here.
Don't have to：表示不必（= needn't）。You don't have to come if you're busy.
4. 表“推测”：Must vs. Might/May/Could vs. Can't
这是考试的重灾区，根据确定性排序：
Must：一定（非常肯定的推测）。
The lights are on. He must be at home.
Might / May / Could：可能（不确定的推测）。
He might be at home, but I'm not sure.
Can't：不可能（否定的推测）。
He can't be at home. I just saw him outside.
四、情态动词 + Have Done (完成式)
这是高级用法，表示对过去发生的事情的推测或评价。
结构
含义
示例
Must have done
过去一定做了……
She didn't answer. She must have been asleep. (她当时一定睡着了。)
Might/May have done
过去可能做了……
He might have forgotten my number. (他当时可能忘了我号码。)
Can't have done
过去不可能做了……
He can't have stolen it. He wasn't there. (他当时不可能偷东西。)
Should have done
本应该做（但没做）
You should have told me earlier. (你本该早点告诉我。)
Could have done
本来能做（但没做）
I could have passed the exam, but I was sick. (我本来能考过的。)
五、学习要点与常见错误
后接动词原形：情态动词后永远跟动词原形，不要加 s, ing, ed 或 to。
❌ He cans swim. / He can swimming.
✅ He can swim.
Must 提问的否定回答：
Must I go now? (我必须现在走吗？)
No, you needn't. / No, you don't have to. (不，不必了。)
(注意：不能回答 No, you mustn't，那是“禁止”的意思)。
Need 的双重身份：
作情态动词：Need I go? (多用于疑问句/否定句，较正式)。
作实义动词：I need to go. (更常用)。
