can/can't
can表示能够做某事。can是不定式 be able to的简写,因此是后面接动词原型。
陈述:This coronavirus can sometimes cause illnesses.
疑问:Can this coronavirus cause illnesses. Yes, it can. No, it can't.
could
could 表示过去能够做某事,是can的过去式。
陈述:She could dance when she was young.
疑问:Could she dance when she was young? Yes, she could. No, she couldn't.
will be able to
will be able to 表示将来能做什么。
陈述句:We will be able to travel around over the world after we grow up.
疑问句:Will we be able to travel around over the world after we grow up? Yes, we will. / No, we won't.
用途-请求许可
(正式)Can I/we borrow your breathing mask?
(非正式)Could I/we use your bathroom?(很礼貌)
(非正式)May I/we make a suggestion?(很礼貌)
允许:
(非正式)Yes, you can/may.
(非正式)Yes, of cause (you can).
(非正式)Yes, sure.
特别注意:请求许可时,could就不是can的过去式,这时回答要用“Yes, you can”,而不是“Yes, you could”。
(正式)Yes, you may.
(正式)Yes, certainly.
不允许:
No, you can't.
No, I'm sorry.
No, I'm afraid not.
No, you may not.
注意:陈述句的may/may not只用于书写各非常正式的情况,如公告:
The student may not take cell phones into the exam. (学生不能携带手机进考场)
总结:can在非正式场景下使用,could在正式场景使用且礼貌,而may同样用于正式且礼貌场合,但非常正式只用may。
have to/need to
have to 和need to同样是情态动词,与其它情态动词不同的是它们像主动词一样,有第三人称形式。
例如:
All the cars have to ride by the left in China.(在中国,所有车都靠右行驶)
We need to go to bed early and get up early.(我们应该早睡早起)
- have to用于不得不做的事情,或因为规则法律而做重要的事情;
【不得不做】She has to get up early tomorrow-the train leave at 6.45 am.
注意:在口语中,常常用 have got to 代替 have to,意思与have to完全一样。
例:I've got to get up early tomorrow to catch the train.(我明天必须早起赶火车)
- need to 用于形容某事是必要或有用的,或都对身体或健康有帮助的事情;
We need to respect everyone.
We need to exercise regularly to keep health.
注:我们可以使用need't来替换 don't need to,例:We needn't drink much.
must
must 形容自己认为很重要,必须做的事情。
We must go to bed early and get up early.(我们须要早睡早起)
We must keep an open mind and ready to receive new evidence. (我们需要保持开放的头脑)
had to / didn't have to
had to 是must 和have to的过去式,表示在过去需要做的事情。
She had to change money when she went to Thailand last year. (去年她去泰国时,她需要兑换货币)
We didn't have to get up early yesterday because out train left at 18:00. (昨天我们不必早起,因为火车在18:00开出)
should(shouldn't) / ought to
should 和 ought to 形容做某事是个好建议,should后面不接to,如:
You should call your mother more often.(你应该多妈妈打电话)
You ought to find out how much it costs.(你应该找出它花了多少)
shouldn't表示某事是个坏建议或危险的主意。
You shouldn't eat a lot of cakes.
情态动词对比
可能100%:must be
可能50%: might be / may be(前者口语,后者更多用在书面语)
不可能50%:might not be / may not be(前者口语,后者更多用在书面语)
不可能100%:can't be
过去能够:could
现在能够: can
以后能够:will be able to
参考资料:《语法俱乐部》、English Grammar App
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