专题七 情态动词和虚拟语气
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一、情态动词
1、表能力 can 表示现在有能力做某事 be able to could
表过去有能力做某事 was/were able to Eg:I am starving to death. I can eat two bowls of rice.
If you have a good sleep, you will be able to work out this problem. 2、表推测（可能性）
表示具体事情发生的可能性的层次比较 词形 must will would should ought to can could may might 肯定式 必定、必然 很可能，大概 可能性比will小 按说应该 按说应该 微弱的可能 或许、也许、也说不定 比may还弱 否定式 不会，不该 语气比won’t弱 不可能 不可能 可能不 比may not还弱 疑问式 会......吗？ 语气比will弱 有可能吗？ 语气比can弱 注：表可能性时：can/could一般用于否定句和疑问句中，表示疑惑、惊讶或不相信
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等，could也可用于肯定句；may/might表可能性不大，不用于疑问句； Eg:That can’t be Mary, for she is in hospital.（表不相信） He might be doing his homework now. They asked if we would do that again. We should arrive home before dark. You must be tired after the long journey. 3、表请求、允许
can/could表请求：could不表过去，只表示委婉、客气的语气，但回答时必须用may/might表请求：might 不表过去，仅表示语气更加委婉。 will/would表请求：用于第二人称的疑问句。 shall表请求：用于第一、三人称的疑问句。 Eg:Can I use your computer for a while?
---could I use your bicycle? ---Yes, you can. /No, you can’t. May/might I make a suggestion? Will you help me with the luggage? Would you pay me in cash, please? Shall he come in or wait outside. 4、表必要性
must 表必要性时，意为“必须”
don’t have to/ needn’t do sth,表示“表示不必要做......” 5、“情态动词+have done”用法一览表。 1) must have done 想必/准是/一定做了 否定式：can’t/couldn’t have done
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can. Eg:It must have rained last night for the road is quite muddy. 2) can/could have done 本来能够做某事却没做/过去可能会做 Eg:Can he have gone to his aunt’s?
3) cannot/couldn’t have done 本不能够做某事却做了/过去不可能做某事 Eg:He cannot have forgotten it.他不可能忘记那件事。
4) may/might have done 也许/或许已经做......(一般只用于肯定句或否定句) Eg:It’s too late. I think he may have gone to bed. He may not have finished the work.
5) should/ought to have done 本该做某事但实际上却没做 Eg:You ought to have done this exercise more carefully. 6) should not/ ought not to have done 本不该做某事但却做了 Eg:You shouldn’t have told her the truth. 7) needn’t have done 本不必要做某事却做了
Eg:You needn’t have taken a taxi here, for it was very near to my house. 6、情态动词的其他用法
1）cannot but do sth 表示“不得不，只好做某事” Eg:I cannot but choose to go. 我只好去。 2）may well/may as well
“may well+动词原形”,意为“完全能，很可能”，相当于to be very likely to Eg:He may well be proud of his son. 他大可为他的儿子感到自豪。 “may as well +动词原形”，意为“最好，满可以，倒不如” Eg:You may as well do it at once. 你最好现在就做这件事。
二、虚拟语气
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1、虚拟语气在非真实条件句的应用
与现在事实相反：条件句：if +主语+动词的过去式（be动词用were） 主句：主语+should/would/could/might+动词原形 Eg:If I were you, I should seize the chance to go abroad. 与过去事实相反：条件句：if+主语+had+过去分词
主句：主语+should/would/could/might+have +过去分词 Eg:If you had taken my advice, you would not have failed in the exams. If+主语+动词的过去式
与将来事实相反：条件句 If+主语+were to +动词原形 If+主语+should +动词原形 主句：主语+should/would/could/might
Eg:If he should not come tomorrow, we should put off the meeting till next Monday. 2、使用虚拟条件句时要注意以下4点：
1）错综时间条件句，动词的形式要根据它所表示的时间做出相应的调整。 Eg:If you had followed my advice just now, you would be better now. If you had studied hard before, you would be a college student now. 2）if 省略句
Eg:If I were at school again (Were I at school again),I would study harder. If you had come earlier (Had you come earlier), you would catch the bus. If it should rain tomorrow (Should it rain tomorrow), we would not go climbing. 3）介词短语代替的条件状语从句:with/without/but for
Eg:Without your help, we couldn’t have finished the work ahead of time. But for your help, ......
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