1、can和could (could为can的过去式) 的基本用法
(1)表示能力,如:He can speak English better than you. (2)在疑问句和否定句中表示怀疑、猜测或可能性,如:Can this green bike be Liu Dongs? (3)表示许可时can可以和may换用,如:You can (may) go home now. (4)如果要表示语气婉转,可用could代替can,这时could不再是can的过去式,如:Could you come again tomorrow? (5)can和be able to都可表示能力,两者在意思上没有什么区别。但是can只能有现在式和过去式,而be able to则有更多的形式,如:He will be able to do the work better.
2、may和might (might为may的过去式)的基本用法
(1)表示允许或征询对方许可,有可以之意,如:You may use my dictionary. 在回答对方说可以做某事或不可以做某事时,一般多不用may或 may not,以避免语气生硬或不容气。而用比较婉转的说法进行回答。如:---- May I use this dictionary? ---- Yes, please. 或 ---- Certainly. 在请求对方许可时,如果Might I? 就比用May I? 语气更婉转些,如:May I have a look at your new computer? 但是表示阻止或禁止对方做某事时,要用must not代替may not,如:---- May we swim in this lake? ---- No, you mustnt. Its too dangerous. (2)may或 might都可以表示可能性,表示或许、可能之意,如果用might表示可能性,则语气更加不肯定,如:They may (might) be in the library now .
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