情态动词¶
助动词¶
助动词本身没有实际意义（无法被翻译），用来帮助谓语动词表达时态、语态、否定和疑问：
| 助动词 | 作用 |
|---|---|
| be | 构成进行时、被动语态 |
| have | 构成完成时 |
| do | 构成一般时态的疑问、否定，或表示强调 |
| shall / will | 构成将来时 |
| should / would | 构成过去将来时 |
Tip
在疑问句和否定句中，普通的实义动词（如 eat、know、like）不能直接加 not，也 不能自己移到句首提问，必须由助动词来完成：
- I don't know. ✓ / I know not. ✗
- Do you like it? ✓ / Like you it? ✗
情态动词¶
情态动词表示说话人的态度，如能力、许可、义务、推测，后接动词原形。
| 情态动词 | 用法 | 例句 |
|---|---|---|
| can | 能力、请求或许可、可能性 | I can swim. Can I borrow your pen? |
| could | 过去的能力；请求时比 can 更礼貌 | Could you help me? |
| may | 请求或许可、可能性（比 can/could 弱）；放在句首表祝愿 | May I come in? May you succeed! |
| might | 可能性，通常比 may 更不确定 | It might rain. |
| must | 必须；mustn't 表禁止 | You must wear a seat belt. |
| will | 将来的意愿、预测；表请求 | I will help you with your homework. It will rain tomorrow. |
| would | 过去的习惯、客气的请求、假设 | When I was a child, I would go fishing every weekend. Would you please lend me that book? |
| shall | 第一人称的将来；提出建议、征求意见 | I shall visit Paris next summer. Shall we go for a walk? |
| should | 应当；有一定根据的推测；用于虚拟语气 | You should eat more vegetables. She should be here by now. |
| ought to | 应该，与 should 相近，但语气更强 | You ought to apologize for your behavior. |
| dare | 敢 | How dare you! |
| need | 需要（多用于否定句和疑问句） | You needn't worry. |
| used to | 过去经常做（现在不做了），强调与现在的对比 | I used to smoke. |
must 表推测¶
must 表推测时只用于肯定句；否定推测用 can't：
- The light is on. Jack must be at home.
- The light isn't on. Jack can't be at home.
起情态作用的短语¶
| 短语 | 含义 |
|---|---|
| be able to | 能够。can 只有现在式和过去式，其他时态要用 be able to，如 will be able to；was/were able to 强调过去成功做成了某事 |
| be going to | 计划做某事，并且很快就会做 |
| be about to | 马上就要做某事 |
| be to | 按计划、安排或命令要做某事 |
| had better（'d better） | 最好做某事，含有忠告、警告的意味 |