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 Jenny’s 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：
It’s a pity that you missed the exciting football match。
It remains 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.
尺度函数4。 It + 不及物动词(seem， appear， happen等) + 主语从句， 这类不及物动词有：appear， happen, seem等， 如：
It seemed that he didn’t 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:
It’s necessary for the young to master two foreign 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：
It’s very kind 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 good, no use, a waste of, useless, senseless等，如：
It’s 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 女行长的沉沦mountain 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. whenever
2. 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. it
3。 is a fact that English is being 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. it
5。 ________ will take you a lot of time to find him in the crowd.
A。 It B。 I C. We D。 They
6。 It is no use his _________ there, the situation is hopeless now。
A. to go B。 to be going C. going D. having gone
7. It is never too late to learn, ________？
A. is it B。 isn’t it C. does it D. doesn't it
8. ___________ 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 China’s Jiangxi Province killed 13 people
A. It B. As C. That D. What
10. ___________ certain that his invention will make people’s life convenient。
A。 That’s B. This is C. It's D。 What’s
11. _________ 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 said
12. __________ that their daughter got an opportunity to study abroad。
A. It’s a exciting news B. This is an exciting news
C。 This is exiting news D. It’s exciting news
参考答案：
1——5 BDDDA 6-—10CACAC 11——12CD
㈠ 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. ⒈ it代替不定式短语 常用于下列句型中：It + be +形容词 + (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 It’s up to sb。 to do sth. 干…是某人的职责或义务 ①It is everyone’s 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）电生理 ②It 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 Chinese）
③It 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）
④It 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sb.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.
①It’s 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。
③It’s kind of you to help me。
感谢你对我的帮助.（= You are kind to help me. ）
④It 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
⒉ 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。
①It is no good/ use having a car if you can’t 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having a car)
②It is a waste of time watching TV。
看电视是浪费时间。（ it作形式主语,代替动名词短语watching TV)
③It is no use asking him.
问他没有用。（ it作形式主语，代替动名词短语asking him）
⒊ it代替主语从句：
这个主语从句可以用连词that引导，也可以用连接代词或连接副词引导.
①It was clear that they had no desire for peace.
很清楚他们没有和平的诚意。
（ it作形式主语,代替主语从句that they had no desire for peace）
②It is doubtful whether she will be able to come。
她能否来还很难说。( it作形式主语，代替主语从句whether she will be able to come)
③It was clear enough what she meant。
她的意思是够清楚的.（ it作形式主语，代替主语从句what she meant）
用it作形式主语，把真正的主语从句移至后面的常见句型有：
◆ It is a pity/ a shame/ a fact/ an honour/ good news/ no wonder/ a wonder… that…
①水葫芦喊冤It was a pity that the engineer couldn’t come。
可惜工程师没能够来。
②It is a wonder that he continues to gamble when he always loses！
令人惊讶的是他一直输还一直赌.
◆ It is + 形容词（如: strange/ natural/ surprising/ obvious/ true/ fortunate/ wonderful/ funny/ possible/ impossible/ likely/ unlikely/ clear/ unusual/ certain/ important…)that …
①It is certain that he will win. 他一定会取胜。。
◆ It seems/ happened/ turned out/ suddenly struck me/ occurred to me…that …
①It happened that I wasn’t there that day。 恰好那天我不在那里。
◆ It + be + 过去分词（如：said/ reported/ announced/ decided/ expected/ hoped/ known/ believed…）that…
②It is said that nothing has been done about it.
据说至今对此没采取任何措施。
③It is reported that more than twenty people were killed in that bus accident.
据报道在那次的公共汽车事故中有二十多人丧生。
* 注意要在It is suggested/required/desired/ordered/proposed…that…的主语从句中用虚拟语气： （should） + 动词原形
①It is requested that he give a performance at the party.
有人请求他在聚会上表演一个节目.
②It is suggested that we should get everything ready by tonight.
建议一切在今晚准备好。
◆ It doesn’t matter +连接代词或副词…
It doesn’t make too much difference +连接代词或副词…
①It doesn’t matter whether he comes or not.
他来还是不来都没关系。
②It won’t make much difference whether you go today or tomorrow。
你今天去也好，明天去也好,关系不大。
㈡ it作形式宾语：
it作形式宾语时，可以代替三种形式：不定式、动名词和宾语从句。
it作形式宾语时,需具备两个条件：① 真正的宾语是不定式、动名词或从句 ② 有宾语补足语 具备了这两个条件，形式宾语it一定要用.
⒈ it代替不定式短语
think/ find/ feel/ consider/ make/ regard…+ it +形容词/名词 + 不定式短语
①I find it pleasant to work with him。
（it 作形式宾语,代替不定式短语to work with him)我觉得和他一起工作很愉快.
②They felt it difficult to finish the work in such a short time。
（ it 作形式宾语，代替不定式短语to finish the work in such a short time）
他们认为在这么短的时间内完成这项工作是困难的.
⑥All these noises made it impossible for me to go on with the work.
（ it 作形式宾语,代替不定式短语to go on with the work)这些噪音使我无法继续工作.
⒉ it代替动名词短语（ 这只限于少数句型,在多数情况下用不定式时更多一些)
①The professor considers it no good reading without understanding.
这位教授认为阅读而不理解没有益处。
③He found it useless( no use) arguing with him.
他发现和他辩论没有用。
时代广场的蟋蟀全文 ④He thought it absolutely senseless attempting the impossible。
想做不可能的事情，他认为是完全没道理的。
⒊ it代替宾语从句：
①We all thought it a pity that the conference should have been cancelled。
会议取消了我们都感到很遗憾。
②I took it for granted that they were not coming. 我想他们准是不会来的了。
③I’ll see to it that everything is ready in time。我负责使一切都按时准备好。
㈢ 含it 的常用句型
英语中有一些固定句型中含有it,应注意这些句型的使用。
⒈ It be + 被强调部分 + that/ who + 其他成分
I met Peter in Japan last year.
⑴ It was I who/that met Peter in Japan last year.
⑵ It was Peter whom/that I met in Japan last year.
⑶ It was in Japan that I met Peter last year.
⑷ It was last year that I met Peter in Japan。
＊ not …until 也可用于强调句型
例１ Mr Brown didn’t come back until eleven o’clock.
可变为:It was not until eleven o’clock that Mr Brown came back。
＊ 强调句也可变为特殊疑问句
例１ It was in 1949 that the PRC was founded.
可变为：When was it that the PRC was founded?
⒉ It’s + 地点状语+ that…（强调句） It’s +地点名词+ where… （定语从句）
①It is the town where I was born.
（the town 为地点名词,定语从句)这是我出生的镇子。
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