Modal verbs are special auxiliary verbs that add meanings like ability, permission, advice, obligation, possibility, or prediction to another verb. They do not usually describe an action by themselves; they modify the main verb. In English, a modal is followed by the base form of a verb, and the pair functions together as the verb phrase of the sentence.

The most common modal verbs are can, could, may, might, must, shall, should, will, and would. These modals are used to express a range of meanings, and context often decides the exact interpretation. Some modals sound more formal or more indirect than others, especially could, might, and would.

Word/Phrase	🇨🇳 中文 单词/短语	Definition	🇨🇳 中文 定义	Example	🇨🇳 中文 示例
can	🔤 可以	ability or permission	🔤 能力或许可	I can swim.	🔤 我会游泳。
could	🔤 could	past ability or polite possibility	🔤 过去的能力或礼貌的可能性	Could you help me?	🔤 你能帮我吗？
may	🔤 may	formal permission or possibility	🔤 正式许可或可能性	You may leave now.	🔤 你现在可以离开。
might	🔤 might	weaker possibility	🔤 较弱的可能性	It might rain later.	🔤 稍后可能下雨。
must	🔤 must	strong obligation or logical conclusion	🔤 强制义务或逻辑推断	You must wear a badge.	🔤 你必须戴胸牌。
should	🔤 should	advice or expectation	🔤 建议或期望	You should rest.	🔤 你应该休息。
will	🔤 will	future or willingness	🔤 将来或意愿	I will call you.	🔤 我会打电话给你。
would	🔤 would	polite request or hypothetical	🔤 礼貌请求或假设	I would like some tea.	🔤 我想要些茶。

A modal verb is followed by the base verb without to: modal + base verb. In negatives, not comes after the modal. In questions, the modal comes before the subject. Modals do not take -s in the third person singular and do not use do or does for questions and negatives.

Rule	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
modal + base verb	📌 modal + base verb	She can drive.	📌 她会开车。
modal + not + base verb	📌 modal + not + base verb	They should not worry.	📌 他们不应该担心。
modal before subject in questions	📌 modal before subject in questions	Can you stay?	📌 你能留下来吗？
no third person -s on modals	📌 no third person -s on modals	He can help.	📌 他可以帮忙。
no do or does with modals	📌 no do or does with modals	Will she come?	📌 将她来吗？

Modals often show how strong a speaker feels about a situation, from weak possibility to strong certainty or obligation. Might and could are usually weaker than may, which is often weaker than must for conclusions. For advice, should is softer than must, and will is stronger than would for willingness or intention.

Can is common for ability and informal permission, while could can describe past ability or a more polite request. May is often used for formal permission and polite permission questions, though it can also express possibility. Choosing between can, could, and may mainly changes formality and indirectness.

Rule	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
ability in the present: can	📌 现在的能力：可以	I can read French.	📌 我会读法语。
ability in the past: could	📌 过去的能力：could	When I was five, I could ride a bike.	📌 当我五岁时，我会骑自行车。
informal permission: can	📌 非正式许可：可以	You can sit here.	📌 你可以坐在这里。
formal permission: may	📌 正式许可：may	You may begin the exam.	📌 你可以开始考试。
polite request: could or may	📌 礼貌请求：could 或 may	Could I ask a question?	📌 我可以提一个问题吗？

Must expresses strong obligation from the speaker or strong rules, and it can also express a strong logical conclusion. Should expresses advice, recommendation, or expectation and is less forceful than must. In many real situations, have to can replace must for obligation, especially when the obligation comes from outside rules rather than the speaker.

Rule	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
strong obligation: must	📌 强烈义务：must	You must stop at red lights.	📌 你必须在红灯前停车。
advice or recommendation: should	📌 建议或推荐：should	You should see a doctor.	📌 你应该看医生。
expectation: should	📌 期望：should	The train should arrive soon.	📌 火车应该很快到达。
strong conclusion: must	📌 强烈结论：must	She must be at work now.	📌 她现在一定在工作。

May, might, and could express possibility, but they usually differ in certainty and tone. Might is often the weakest and most tentative, while may can sound a bit more confident or formal. Could often suggests a possible option among others rather than a prediction.

Rule	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
tentative possibility: might	📌 暂时性可能：might	I might go later.	📌 我可能晚些时候去。
possibility, often more formal: may	📌 可能性，通常更正式：may	We may need more time.	📌 我们可能需要更多时间。
one possible option: could	📌 一个可能的选项：could	We could take the train.	📌 我们可以坐火车去。

Will is used for future reference, decisions at the moment of speaking, promises, and willingness. Shall is limited in modern English and is most common in questions offering or suggesting, especially in British English. Would is used for polite requests, offers, and more indirect statements about willingness.

Rule	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
future decision or plan: will	📌 将来决定或计划：will	I will email you tonight.	📌 我今晚会给你发邮件。
promise: will	📌 承诺：will	I will be there on time.	📌 我会准时到达。
offer or suggestion question: shall	📌 礼貌请求：shall	Shall we start?	📌 我们开始好吗？
polite request: would	📌 礼貌请求：would	Would you open the window?	📌 你愿意打开窗户吗？
indirect willingness: would	📌 间接的意愿：would	I would help if I could.	📌 如果可以，我会帮忙。

Would and could are common for hypothetical situations and conditional meaning. Would often describes what happens in an imagined situation, while could often shows hypothetical ability or possibility. In conditional sentences, would and could frequently appear in the result clause to show that the situation is not certain or is not real now.

Modals have limited past forms, so English often uses modal + have + past participle to talk about past possibility, past certainty, or past obligation. This structure is used to comment on past events, often with meanings like inference, regret, or criticism. Not all modals use perfect forms equally, but could have, might have, should have, would have, and must have are common.

Rule	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
past possibility: might have + past participle	📌 过去的可能性：might have + 过去分词	She might have missed the bus.	📌 她可能错过了公交车。
past possibility or ability: could have + past participle	📌 过去的可能性或能力：could have + 过去分词	I could have called you.	📌 我本可以给你打电话。
past advice or criticism: should have + past participle	📌 过去的建议或批评：should have + 过去分词	You should have told me.	📌 你本应该告诉我。
past inference: must have + past participle	📌 过去的推断：must have + 过去分词	They must have left early.	📌 他们一定早就走了。
hypothetical past result: would have + past participle	📌 假设的过去结果：would have + 过去分词	I would have gone if I had known.	📌 如果我早点知道我就会去。

Some verbs act like modals in meaning but behave more like normal verbs in grammar, especially have to, be able to, ought to, and need to. They can often express similar ideas to must, can, or should, but they use to and can change with tense in more regular ways. Learning semi-modals helps you express modal meanings in tenses where core modals are limited.

Word/Phrase	🇨🇳 中文 单词/短语	Definition	🇨🇳 中文 定义	Example	🇨🇳 中文 示例
have to	🔤 have to	external obligation or requirement	🔤 外部义务或要求	I have to work today.	🔤 我今天必须工作。
be able to	🔤 be able to	ability with flexible tense	🔤 能力，时态灵活	She was able to finish.	🔤 她能够完成。
ought to	🔤 ought to	advice or expectation, formal	🔤 建议或期望，正式	You ought to apologize.	🔤 你应该道歉。
need to	🔤 need to	necessity or requirement	🔤 必要性或要求	We need to leave now.	🔤 我们现在需要离开。