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1、 Unit 21 语法一、 虚拟语气条件状语从句种类及用法 条件句类型条件句主句零条件句表示普遍真理和客观事实一般现在时一般现在时第一条件句依据将来事件A, B事件很可能在将来发生一般现在时一般将来时第二条件句与现在事实相反If+主语+动词的过去式（be动词用were）主语+should/would/ could/might+动词原形与将来事实相反1.If+主语+动词过去式主语+should/would/ could /might +动词原形第三条件句与过去事实相反If+主语+had+过去分词主语+should/would/could/might+have done1零条件句。表示普遍真理和客
2、观事实。其主句、从句都用一般现在时If you watch too much TV, you eyes hurt.If we eat too much sweet, your teeth become weak. If my heart stops beating, I die.2第一条件句。依据将来事件A, B事件很可能在将来发生。主句用一般将来时，从句都用一般现在时。即：主将从现。If I enter Tsinghua University, my mum will buy a BMW for me.If my English teacher criticizes me tomorrow,
3、 I will have a talk with the headmaster.3与现在事实相反的虚拟语气eg. If I were a girl, I should be a beauty. I would buy two cars if I had one million yuan. 4与将来事实相反的虚拟语气eg：If I should take Shenzhou Ten to the moon, I would dance with ChangE. If I were to become President of the United States, I would not start
4、 a war. 5与过去事实相反的虚拟语气eg：If he had seen you yesterday, he would have asked you about it. If you had come earlier, you would (might) have met him. I should (would) have called you if I had known your telephone number.6. 混合虚拟条件句以上虚拟语气的句子都有固定形式，不难掌握。难就难在准确判定与事实相反的时间类别。混合虚拟条件句，即从句与主句的动作不是同时发生。这时就要分别根据各自发
5、生的时间，选用相对的虚拟形式。eg：If he had followed the teachers advice, he would not be so heart-broken right now. 与过去事实相反 与现在事实相反If China had not been liberated, the working people would still lead a miserable life now. 与过去事实相反 与现在事实相反7. 用介词短语等代替if 虚拟条件句。有时虚拟条件不用if从句表示，而是用介词短语 with, without, but for 或副词otherwise
6、,连词表示。eg：With your assistance (=If we had your assistance), we might finish the plan earlier.如果有你的威胁，我们会早一点完成计划。I could not have lived through Christmas without giving you that present. 如果没给你买那件礼物，我圣诞节都会过不好。 I was busy last week, otherwise I would have come to see you. 上周我很忙， 否则我会去看你的。8. 省略if后的倒装如果i
7、f条件从句中谓语部分含有were或者had, should词。把were或者had, should 放到主语前面，可以省去if。 所表达的意思和带有if的句子意思相同。eg： If I were a girl, I would wear short hair. 如果我是女生， 我就留短发。= Were I a girl, I would wear short hair.If I should take Shenzhou 10 to the moon, I would invite Chang-O to have a dance with me.= Should I take Shenzhou
8、10 to the moon, I would invite Chang-O to have a dance with me.If Peter hadnt fallen in love with Cuihua, he might have been admitted to a key university.= Had Peter not fallen in love with Cuihua, he might have been admitted to a key university.如果彼得未和翠花堕入爱河， 彼得可能会被一所重点大学录取。二、表示推测的情态动词 1.Can 最常用于疑问句
9、和否定句。疑问句表示“可能” 在否定句中是must的否定形式 “肯定不” 在肯定句，表示客观（理论上）的可能性,常常可以说明人或事物的特性。并不牵涉到某件事是否真的会发生. 当我们想要表达发生某事的实际可能性时，我们用 could（或 may, 或 might）。（*can一般用于疑问句和否定句中表示具体事情实际发生的可能性）。 eg. Everyone can make mistakes in his life. 每个人在一生中都有可能犯错误。 Scotland can be very warm in September. 九月的苏格兰可以非常暖和。 He is in poor health
10、. He can be ill at any time. 他身体不好，随时都可能会生病。 Can the news be true? 这消息可能是真的吗？2.Could 用于肯定句，表示某事仅仅有发生的可能性，并不表示特别可能发生即“微弱的可能”。还表示将来可能。eg.We could go climbing this summer, but I doubt if well have time. It could rain later on this evening. Could 疑问句、否定句的用法与can相同， 但语气比can委婉、微弱。 eg. The climate change co
11、uld/can bring the planet to the edge of disaster. You mustnt smoke while you are walking around in the woods. You could start a fire. We could may，might go to Japan this summer. 今年夏天我们可能要去日本。(将来可能性) 3.May 用于肯定句 表示具体事情的发生的可能性。“可能” 、“或许”、“也说不定”May not 可能不May 不用于疑问句中 （疑问句中用can） eg. The light isnt on. I
12、t may be broken. They may not be there today.4.Might 在表示可能这个概念上，may和might是可以互换的，但might的语气比may表示的可能性更小，语气更委婉。. eg. I think you might like something to read, so I have brought you some books. 5.Must 用于肯定句 “肯定” 。 不用于否定句和疑问句。 否定句中用 cant eg. You havent eaten anything since morning; you must be hungry. 6.
13、 need 作情态动词用时, 常用于疑问句、否定句。 （dare的用法相同）- Need I finish the work today?- Yes, you must. / No, you neednt. neednt=dont have to 没有必要 *need 作实意动词讲，可用于肯、否、疑句中。随主语的人称和数而变化。过去式和pp为needed.He has grown up. We dont have to worry about him. (实意动词）=We neednt worry about him.Need have done 需要做某事，可没做Neednt have do
14、ne 没有必要做某事，可是做了。 should have done sth 过去应该做某事。表批评。7.will1.用于第2人称疑问句中。will表示请求、建议等，would更委婉。Will you pass me the ball, please?2.will表示意志、愿望和决心。肯定句中用于各种人称。I will never do that again.I will help you.3. 表示推测。 will用于谈论现在，would可用于谈论过去，也可用于谈论现在(语气较委婉)。如：【注】有时可后接完成式，表示对过去可能已经发生的情况进行推测。如： This car will hold six people. 这辆小汽车可以坐六个人。 Ask him. He will know. 问问他吧，他大概知道。 You will have heard of it. 你已经听到那事了吧。 Many people will have watched 3D Titanic.8. 情态动词+have done 对过去的推测must have done 肯定性推测，用于肯定句中cantcouldnt have done 最常用于否定句。是must的否定形式。“肯定不”。（有时用在疑问句中，表示对过去的可能性推测）。maymight have do
