1、must
(1) must 表示主观看法,意为“必须”。
例如:
You must stay here until I come back.
Must I hand in my homework right now?
对must引导的疑问句,肯定回答为must,否定回答为needn't 或don't have to。
例如:
— Must I finish my homework?
— No, you needn't.
(2) must也可以表示有把握的推测,意为“ 一定,肯定”,用于肯定句。
例如:
The light is on, so he must be at home now.
其否定形式mustn't表示“禁止,不许”。
例如:
You mustn't play with fire。
You mustn't be late。
2、could
(1) can的过去式,意为“能、会”,表示过去的能力。
例如:
He could write poems when he was 10.
(2) could在疑问句中,表示委婉请求的语气,此时could没有过去式的意思。
例如:
Could you do me a favour?
— Could I use your pen?
— Yes, you can.(注意回答)
3、might
might为may的过去式。might表示推测时,表示可能性低于may(此时might没有过去式的意思),当请求讲时,比may的语气更委婉。
He is away from school. He might be sick.
Might I use your dictionary?
4、can
(1) 表示能力,一般译为“能、会”,尤其指生来具备的能力。
例如:She can swim fast, but I can't.
(2) 表示许可,常在口语中。
例如:You can use my dictionary.
(3) 表示推测,意为“可能”,常用于否定句和疑问句中,此时can't译为“不可能”。
例如:
— Can the news be true?
— No, it can't be our teacher. He is on a visit to the Great Wall.