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1、It作形式主语常见句型动词不定式，动名词或主语从句作主语时，通常把它们放在谓语动词前，而用it作形式主语，放到句首，常见句型有：1. It + be + 形容词+主语从句，如：It is uncertain whether he can come to Jennys birthday party or not.2. It + be + 名词词组 + 主语从句，这类名词有：a fact, a pity, a shame, an honour，a question等，如：Its a pity that you missed the exciting football match.It remain
2、s a question whether he will come or not. 3. It + be + 过去分词 + 主语从句，这类过去分词有：announced, believed, expected, hoped, decided, reported, said, shown等， 如：It is reported that 16 people were killed in the earthquake.It is not decided who will give the operation to the patient.4. It + 不及物动词(seem, appear, hap
3、pen等) + 主语从句, 这类不及物动词有：appear, happen, seem等， 如：It seemed that he didnt tell the truth.It happened that I was out when he called.5. It + be + 形容词 + for sb.）+ 动词不定式这类句型常用形容词easy, difficult, hard, important, possible，wise等作表语，有时候为了强调不定式动作的执行者，常在不定式前加for sb，如：Its necessary for the young to master two f
4、oreign language.It is unwise to give the children whatever they want.这类形容词有：easy, difficult, hard, important, possible，wise 6. It + be + 形容词 + of sb. + 动词不定式，这类形容词常是表示心理品质,性格特征的形容词，如：kind, nice stupid, clever, foolish, polite, impolite, silly, selfish, considerate等。某人和这些形容词可以构成主系表结构。如：Its very kind
5、of you to help me with the work.=You are kind to help me.It seemed selfish of him not to share his dictionary with others.=He seemed selfish not to share his dictionary with others.7. It + be +名词词组 + 动词不定式，如：It is not a good habit to stay up too late.8. It + be + 名词或形容词 + 动名词，这类名词和形容词常常是：good, no go
6、od, no use, a waste of, useless, senseless等，如：Its a waste of time talking to her any more.It is no use arguing about the matter with him.9. It + take ( sb. )+ 时间（金钱）+ 动词不定式，如：It took the workers almost three years to finish building the dam.It will take a whole day to get to the top of the mountain
7、on foot.实战演练：1. It is generally considered unwise to give a child he or she wants. (NMET 1997)A. however B. whatever C. whichever D. whenever2. In fact is a hard job for the police to keep order in an important football match. (2001 上海)A. this B. that C. there D. it3. is a fact that English is being
8、 accepted as an international language. (NMET1995)A. There B. This C. That D. It 4. Is necessary to take off our shoes when we enter the lab? (MET88) A. everyone B. this C. her D. it5. _ will take you a lot of time to find him in the crowd. A. It B. I C. We D. They6. It is no use his _ there, the si
9、tuation is hopeless now. A. to go B. to be going C. going D. having gone7. It is never too late to learn, _? A. is it B. isnt it C. does it D. doesnt it8. _ is known to all that Shenzhou VI Launched Successfully. A. As B. What C. It D. That 9. _ is reported in the newspaper, the Earthquake in east C
10、hinas Jiangxi Province killed 13 people A. It B. As C. That D. What10. _ certain that his invention will make peoples life convenient. A. Thats B. This is C. Its D. Whats11. _ that there is another football match on the air this evening. A. It says B. It was said C. It is said D. What was said12. _
11、that their daughter got an opportunity to study abroad. A. Its a exciting news B. This is an exciting news C. This is exiting news D. Its exciting news参考答案：15 BDDDA 610CACAC 1112CD it作形式主语：it作形式主语时没有具体的意义，而只是帮助把真正的主语移到句子后部去，使句子显得平稳一些。it作形式主语时，可以代替三种形式：不定式、动名词和主语从句。 it代替不定式短语常用于下列句型中：It + be +形容词 + (
12、for/of sb.) to do sth.It + be + 名词 + to do sth.It takes sb. some time to do sth. 干花了某人时间Its up to sb. to do sth. 干是某人的职责或义务It is everyones duty to obey the law.遵守法律是每个人的义务。（ it作形式主语, 代替不定式短语to obey the law）It is difficult for a foreigner to learn Chinese.对于一个外国人来说，学习汉语是困难的。（ it作形式主语, 代替不定式短语to learn
13、 Chinese）It is not right to use these places as rubbish dumps.把这些地方用来倒垃圾是不对的。（it作形式主语, 代替不定式短语to use these places as rubbish dumps）It took them a year to build the bridge.建这座桥花了他们一年的时间。（ it作形式主语, 代替不定式短语to build the bridge） It be + 形容词+ for sb. to do sth.与It be + 形容词+of sb. to do sth.：如果句型中的形容词描述的是s
14、b.的品质、品格，在逻辑上可以作sb.的表语，则sb.前应用介词of，否则就用for。Its necessary for us to use a short-wave radio to pick up the programmes.我们用一台短波收音机收听这些节目是必要的。Its kind of you to help me.感谢你对我的帮助。（= You are kind to help me. ）It was foolish of him to give up the job.他放弃那工作是愚蠢的。（= He was foolish to give up the job it代替动名词短语：动名词作主语用后置形式的情况远不及不定式那么普遍，常限于no good, no use, useless, waste等词的后面。 It is no good/ use having a car if you cant drive.如果你不会开车，有车也没用。( it作形式主语,代替动名词短语hav
