核心情态动词：
can, could, may, might, will, would, shall, should, must
半情态动词（具有情态含义，但用法稍有不同）：
ought to, need, dare (较少用), have (got) to, used to
情态动词的主要特点：
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没有第三人称单数变化： 如：He can... / She must... (不是 cans / musts)
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不接"to"构成不定式： 如：I can swim. (不是 can to swim) / She must study. (不是 musts to study)
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不构成进行时或完成时形式： 自身没有-ing形式或-ed形式。
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构成疑问句和否定句不加助动词： 情态动词本身就能构成疑问和否定。
- 疑问句: Can you...? / Must he...?
- 否定句: cannot/can't... / must not/mustn't...
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后接动词原形： 直接加在主要动词的原形前。如：I should go. / They might come.
核心用法概述表：
| 情态动词 | 核心含义 | 主要用法举例 |
|---|---|---|
| can | 能力，许可，可能性（较肯定） | I can swim.（能力） You can leave now.（许可） It can be hot here in summer.（可能性） |
| could | can的过去式，礼貌请求，较弱可能性 | When I was young, I could run fast.（过去能力） Could you open the window?（礼貌请求） It could rain later.（较弱可能性） |
| may | 正式许可，可能性（比might稍肯定） | May I sit here?（正式许可） It may snow tomorrow.（可能性） |
| might | 可能性（比may更不确定），建议（比may弱） | She might be late.（很不确定的可能） You might try restarting the computer.（建议）Might I ask a question?（非常礼貌/正式的请求） |
| will | 未来（决心/预测），意愿，习惯性行为 | I will help you.（决心/承诺） The sun will rise at 6am tomorrow.（预测） Will you marry me?（意愿请求） Every Saturday he will go fishing.（习惯性） |
| would | will的过去式，礼貌请求，假设，过去习惯 | He said he would come.（过去将来） Would you mind helping me?（礼貌请求） If I had time, I would travel more.（假设条件句） She would often visit her grandma.（过去习惯） |
| shall | 正式提议、建议、义务（多用于英式英语） | Shall I open the window?（提议） The rules state that players shall wear uniforms.（正式规定/义务） |
| should | 建议，义务（比must弱），预期 | You should see a doctor.（建议） We should respect our elders.（义务） He should arrive soon.（预期/预测） |
| must | 强烈的义务/必要性，逻辑必然性（推测） | You must stop at a red light.（强烈义务） He worked all night; he must be tired.（逻辑推测，几乎肯定） |
| ought to | 应尽义务/建议（比should正式，意义类似） | You ought to apologize.（义务/建议） |
| need | 必要性（疑问/否定句中是情态；肯定句中需用实义动词） | Need I say more?（疑问句，情态） You needn't hurry.（否定句，情态） You need to study hard.（肯定句，实义动词） |
| have (got) to | 义务（多指外部规则/情境），必要性（口语常用替代must） | I have to go to work now.（义务/必要） Do I have to finish this today?（询问必要） |
| used to | 过去经常的状态/习惯（现在不如此） | I used to smoke.（过去习惯，现在不了） There used to be a bookstore here.（过去状态） |
深入解析关键用法和区别：
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能力 (Ability):
can: 现在或一般能力。 (e.g., Birds can fly.)could: 过去能力。 (e.g., I could play piano when I was a child.)be able to: 可以替代can/could，尤其用于表示将来能力或将能力作为成就时。 (e.g., Will you be able to come tomorrow? / He was able to escape.)
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许可 (Permission):
- 请求许可：
Can I...?(最口语),Could I...?(更礼貌),May I...?(更正式),Might I...?(非常正式/礼貌) - 给予许可：
can/may(e.g., Yes, you can/may go now.)may更正式。 - 拒绝许可：
can't/mustn't/may not. (e.g., You can't/mustn't/may not smoke here.) - 过去许可：通常不用
could表示许可被给予（could主要用于请求）。 (可以说：I was allowed to stay up late.)
- 请求许可：
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可能性 (Possibility):
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从强到弱：
- 很强:
must(逻辑推断，几乎肯定) (e.g., The lights are on; he must be home.) - 可能 / 经常:
can(e.g., Driving can be dangerous.)It can be...结构表示“有时会发生”。 - 一般可能:
may/could(e.g., She may/may not come. / It could/might rain.) - 较弱可能:
might(最不确定) (e.g., She might be angry, but I'm not sure.)
- 很强:
-
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义务 (Obligation) 和 必要性 (Necessity):
- 最强：
must(说话人的权威/内在强制力),have (got) to(外部规则/情境要求) (e.g., I must stop smoking. / I have to pay taxes.) 口语中have to常取代must。 - 较强：
need to(作为实义动词) (e.g., You need to be on time.) - 较弱 / 建议：
should/ought to(e.g., You should/ought to apologize.) - 禁止：
mustn't(禁止，不准) (e.g., You mustn't cheat.) /can't(e.g., You can't park here.) - 没义务/没必要:
needn't/don't have to(e.g., You needn't/don't have to come early, dinner is at 8.)
- 最强：
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建议 (Advice):
should/ought to(最常用) (e.g., You should/ought to see a doctor.)could/might(更委婉的建议) (e.g., If you're cold, you could/might close the window.)had better(常带警告意味) (e.g., You'd better study or you'll fail the exam.)
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请求 (Requests):
Can you...?(口语常用)Could you...?(更礼貌)Will you...?(也可以表示请求帮助)Would you...?/Would you mind...?(最礼貌常用)May I...?/Might I...?(请求许可)
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习惯 (Habits):
will(表示一贯的、带点固执或可预测的习惯行为，常带时间状语) (e.g., Every Sunday he will sit in the park and read.)would(表示过去的习惯或反复发生的动作) (e.g., When we were kids, we would spend our summers at the beach.)used to(表示过去的习惯或状态，但现在已经不存在了) (e.g., I used to smoke. / There used to be a cinema here.)
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条件句 (Conditionals):
- 在虚拟条件句中（表示与事实相反或可能性很小），主句常用
would/could/might+ 动词原形。 (e.g., If I were rich, I would travel the world. / If you had studied harder, you could have passed.)
- 在虚拟条件句中（表示与事实相反或可能性很小），主句常用
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推测 (Deduction):
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对现在或一般情况推测：
- 肯定：
must>should>ought to>may>might>could - 否定：
can't>couldn't(几乎不可能),may not/might not(可能不)
- 肯定：
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对过去情况推测：
must/can't/should/may/might/could+have+ 过去分词。 (e.g., The road is wet; it must have rained last night. / She can't have left already; her car is still here.)
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重要提示：
- 语境为王： 情态动词的具体含义和强度非常依赖上下文。比如
must在“You must see this movie!”中表达强烈的建议，而在“Candidates must submit the application by Friday”中表示强制义务。 - 语气差异： 不同情态动词可以传达说话者的不同态度（直接、礼貌、犹豫、强硬等）。
- 地区差异：
shall在英式英语和美式英语中的使用频率有差异（美式英语较少用shall表示将来，多用will）。 - 否定形式： 注意否定形式的含义变化。
mustn't(禁止) 与don't have to(没必要) 意思完全不同。
