it句型根据其用法特点可归纳为２０个句型．

1. It is + 被强调部分 + that ...　 　

　 该句型是强调句型。将被强调的部分放在前面，其它部分置于that之后。被强调部分可以是主语，宾语，表语或状语。强调的主语如果是人，that可以由who换用。如果把这种句型结构划掉后，应该是一个完整无缺的句子。这也是判断强调句型与其它从句的方法。

It was about 600 years ago that the first clock with a face and an hour hand was made.
It was they that (who ) cleaned the classroom yesterday.
It was in the street that I met her father.

2. It is not until + 被强调部分 + that ...

该句型也是强调句型。主要用于强凋时间状语，译成汉语＂直到．．．才．．．＂，可以说是 not ... until ... 的强调形式。

It was not until she took off her dark glasses that I realized she was a famous film star.
= Not until she took off her dark glasses did I realize she was a famous film star.
= I didn’t realize she was a famous film star until she took off her dark glasses.

3. It is clear ( obvious, true, possible, certain....) that .....

　 该句型中it 是形式主语，真正的主语是that 引导的主语从句，常译为＂清楚（显然，真的，肯定．．．）＂　是主语从句最常见的一种结构。 　　

It is very clear that he’s round and tall like a tree. 　　
= That he’s round and tall like a tree is very clear.

4. It is important ( necessary, right, strange, natural...) that ...

　　该句型和上一个同属一个句型。由于主句中的形容词不同，that 后的从句中要用虚拟语气（should + 动词原形），should 可以省去。建议记住该句型中的形容词。 　　

It is important that we (should) learn English well. 　　
It is necessary that he (should) remember these words.

5. It is said (reported, learned....) that ...

　　该句型中的it 仍是形式主语，真正主语是that 引导的主语从句。该结构常译为＂据说（据报道，据悉．．．）＂。 　　

It is said that he has come to Beijing. 　　
It is reported that another earth satellite has been put into orbit.

6. It is suggested ( ordered ... ) that ...

　　该句型和上一个同属一个句型。主句中的过去分词是表示请求，建议，命令等词时，that　后的从句要用虚拟语气（should + 动词原形），should 可以省。常译为＂据建议；有命令．．．） 　　

It is suggested that the meeting ( should ) be put off. 　
It was ordered that we ( should ) arrive there in two hours.

7. It is a pity ( a shame ... ) that ...

该句型中，that后的从句一般用虚拟语气（should + 动词原形），should可省去．表示出乎意料，常译为＂竟然＂。没有这种意义时，则不用虚拟语气。

It is a pity that such a thing ( should ) happen in your class.
这种事竟然发生在你们班上，真是遗憾！ 　
It is a pity that he is ill. 他生病了，真遗憾！

8. It is time ( about time ,high time ) that ...

　　该句型中that 后的从句应该用虚拟语气，值得注意的是① 常用过去时态表示虚拟．② 有时也用should + 动词原形，should 不能省。常译为＂是（正是）．．．的时侯．．．＂。 　　

It is time that children should go to bed. 　　
= It is time that children went to bed.

9. It is the first ( second ... ) time that ...

　　该句型要和上一个句型区别开来。该句型中的 that 从句不用虚拟语气，而用完成时态。至于用什么完成时态，由主句的谓语动词的时态决定。如果是一般现在时，后面从句用现在完成时态；如果是一般过去时，后面从句则用过去完成时态。该结构中 that 可以省去；it有时用 this 替换．常译为＂是第一（二）．．．次．．．＂。

It is the first time I have been here.
= This is the first time I have been here.

10. It is .... since ...

　　该句型主要用作处理瞬间动词的完成时，又要和表示一段时间的时间状语连用的问题。主句中是时间作表语，其时态是现在时或完成时，since 引导的从句通常是一般过去时态，而且是瞬间动词。如果主句是一般过去时，从句则用过去完成时。 　　

It is ( has been ) 5 years since his father died.

11. It is ... when ...

　　该句型中的 when 引导的是一个时间状语从句，主句中的 it 指时间，表语由具体的时间充当。常译为＂当．．．的时候，是．．．＂。 　　

It was 5 o’clock when he came here.

12. It be ... before ...

　　该句型主句中的 it 指时间, 主句中的时态常是将来一般时或过去时两种时态．主句中的表语多是long, not long , 3 days , 2 weeks 等表示时间段的词或短语。常译为＂．．．之后．．．＂。

It was 3 days before he went to Beijing.
It will be not long before he finishes his job.

13. It happens (seems, looks, appears ) that...

　　该句型中it是形式主语，that 引导的是主语从句，主句中的happen , seem等词是不及物动词． 　　

It happened ( so happened ) that he met his teacher in the street. 碰巧．． 　
It seems that he will be back in a few days. 看来．．．

14. It takes sb. ... to do sth.

　　该句型中的不定式是真正的主语，it是形式主语，句型中的直接宾语是时间。常译为＂做．．．要花费某人．．．＂。 　　

It took thousands of people many years to build the Great Wall.

15. It is no good (use ) doing sth.

　　该句型中的真正主语是动名词短语，主句中的表语可以是no good , ( not any good ) , no use , ( not any use )。 　　

It is no good learning English without speaking English.

16. It doesn’t matter whether ( if ) ...

　　该句型中whether(if) 引导的从句是真正主语，该句型常译为　＂不论（是否）．．．没关系．．．。 　　　
It doesn’t matter if they are old.

17. It is kind ( of sb. ) to do sth.

该句型中的不定式短语是真正主语，如果不定式的逻辑主语是由 of引起，主句中的形容词必须是能表示逻辑主语特征的形容词。常见的有： bad , brave , careless, clever , cruel , foolish , good （好心的）, honest , horrible , kind , lazy , modest , naughty , nice（有教养的）, polite, rude , silly , stupid , wise , wrong（错误的）等。 这个句型可以改写为：sb. is kind to do sth. 　

It is kind of you to say so.
= You are kind to say so.

18. It is necessary ( for sb. ) to do sth.

　　该句型与上一个同属一个句型。如果不定式的逻辑主语是由for引起，主句中的形容词通常是表示重要性，紧迫性，频繁程度，难易，安全等情况的形容词。常见的形容词有：① important, necessary, natural ② easy , safe , common , normal , hard , difficult , dangerous , unusual, rare , impossible , pleasant等。 　　
在①中的形容词作表语可以用从句改写， 如： 　　
It is important for her to come to the party.
= It is important that she (should ) come to the party

19. It looks ( seems ) as if ...

　　该句型中it无意义。 as if 引导一个状语从句。常译为，＂看起来好象．．．＂如果与事实不相符合，则用虚拟语气．

It looks as if he is ill.（真的病了）　
It looks as if he were ill. （没有生病）
It seemed as if he were dying.

20. We think it important to learn a foreign language.

　　该句型中的it 作形式宾语。为了记忆方便我们可称该句型为"6123结构"。
　　6指主句中常用的动词：think, believe, make, find, consider, feel；
　　1指的是形式宾语it； 　
　　2指的是宾补的两种形式：形容词或名词；
　　3指的是真正宾语的三种形式：不定式短语，动名词短语或that引导的宾语从句。 　

We think it our duty to clean our classroom every day. 　
He felt it important learning English well. 　
They found it difficult that they would finish their work in two days.

[ 本帖最后由 john2005 于 2007-4-7 19:01 编辑 ]

代词在阅读过程中是难点之一。在英语中，为了语言的精练，避免重复，除了采用“省略”手段外，还广泛地用代词来代替上下文中已经出现过的词或提到的内容。在阅读过程中必须根据上下文的逻辑关系正确判断代词所代替的具体内容，如分析不当，往往会产生“张冠李戴”的错误。必须指出的是，在各种代词中it的用法最为复杂，it用作形式主语或形式宾语时，只有语法意义而没有词汇意义，可以说是“虚”的。但用作代词时，代替某一词或某一具体内容，可以说是“实”的。it在句中表现得“真真假假，虚虚实实”，这给阅读理解造成了很大困难，必须认真分析和辨别。
??为了语言精练，避免重复的目的，除了采用省略和代词外，还采用另一种手段——近义词(带定冠词the的近义词)来替代上文中已出现过的词，这种近义词可称为“替换词”。
??英语中常见的代词有：one(ones)，that，they，those，the former，the latter等。“的用法较为复杂，特别加以归纳。

it的用法
(1)it作无人称代词：it作无人称代词表示自然现象、气候、时间、距离等。it是形式主语没有词汇意义。如：
1)It is very cold today．(气候)
2)It is three o'clock．(时间)
3)It is along way from here．(距离)
(2)it作人称代词：it作人称代词时，用来代替上下文中提到的事或物，有时可代替整个句子的内容。
(3)his(was)+形容词十不定式(to do)，it为形式主语，不定式短语为真正主语。
(过去分词)
(4)his+{形容词}+that(what，how，whether．．．)，it为形式主语，that引导的为主语从句。
(名词)
(5)It+不及物谓语动词+that引导的主语从句。这些动词有：seem，appear，turn out，follow，happen等。
(6)It may well be that引导的主语从句。
(7)it作形式宾语：
1)某些及物动词+it+形容词十不定式；
2)某些及物动词+it+形容词+that...
在上述两种句型中，it是形式宾语，真正的宾语是不定式短语，或that引导的宾语从句。
(8)it用于强调句型：
It is(was)十被强调的成分+that(which，who)
强调句型是简单句，可以用来强调句中主语、宾语和状语，但不能强调定语和谓语。在这种句型中把his．．．that去掉，句子仍成立。

代词在句中分析与翻译的实例
1．All the various transitions from one form of motion into another are governed by one ofthe fundamental laws of nature，that of the eternity of matter and its motion．
运动可以由一种形式变为另一种形式，所有运动形式的变换都取决于一个基本的自然规律，即物质及其运动永恒性的规律。(that代替one，one代替law。)

2．The author enumerate some of the factors which make it difficult for the sociologist to conduct experiments in the same way as his colleagues in the physical sciences．
作者列举了某些使社会科学家难以进行实验的因素，而在同样的情况下，他的自然科学的同事们却可以进行实验。(it为形式宾语代替真正宾语to conduct．．．。)

3．The metallurgist has recently found a metal skin for missiles and supersonic planes，one that will retain its strength at the high temperature caused by air friction．
冶金学家最近发明了一种用于导弹和超音速飞机的金属外壳，这种外壳在大气摩擦引起的高温下仍然能保持其强度。(one作同位语，代替skin。)

4．If one can scale down all dimensions to，let us say，one tenth，the average lengths of the current-paths will be reduced to one tenth．And the speed of operation will be scaled up 10 times．
如果人们能把所有尺寸缩小，比如说，减小到原来的1／10，那么电流路径的平均长度也将减小到1／10，操作速度因而就会按比例提高10倍。(one泛指人。)

5．Underground water reserves are much larger than those on the surface，but as they are unseen we tend to understimate them．
地下水的储量要比地面水大得多，但是由于看不到，所以往往低估了地下水的储量。(those代替reserves。as引导原因状语从句。them代替reserves。)

6．Here we find that the term work has been given by science a somewhat more limited meaning than that to which we have been accustomed．
这里我们发现，科学赋予“功”这个词的含义比我们所习惯的“工作”这个词的含义多少要狭窄一些。(第一个that是连词，引导宾语从句。第二个that是代词，代替work。)

7．We know that air can be compressed， and it is reasonable to suppose that the air at the surface of the earth is compressed by the weight of all the air on top of it．
我们知道空气是可以压缩的，因而我们有理由假设：地球表面的空气受到该空气层上部所有空气重量的压缩。(第一个it是形式主语，代替不定式短语to suppose...。句末的it是人称代词，代替“地球表面上的空气”。)

8．It follows from the principle of relativity that Newton’s second law of motion，although fundamental in classical mechanics，only has an approximate and by no means complete validity．
由相对论原理可知，牛顿运动第二定律尽管是古典力学中的基本原理，但只是近似的，而决不是完全正确的。(It follows from．．．that....，“由……可以得出……”。It是形式主语，代替that引出的主语从句。)

9．It was supposed that bodies fall to the ground with a speed which is proportional to their weight。This means that if bodies of 100 pounds and 1 pound are pushed over a cliff，the former falls 100 times as fast as the latter．
曾经有人设想，物体落地的速度与其重量成正比，这就意味着，如果把重100磅的物体和重l磅的物体从悬崖上推下去，前者下落速度是后者的100倍。(the former指bodies of 100 pounds，the latter指bodies of 1 pound。)

10 ．Even though the same computer might have performed the same operation millions of times in succession， it must still be told exactly how to do that operation every time it repeats it．
即使同一台计算机很可能已经连续完成同样的运算达数百万次之多，但每当它重复这一运算时都得准确地告诉它如何去完成这一运算。(句中有三个it,第一和第二个it都代替计算机，第三个it代替operation。might have performed是虚拟语气，表示对过去发生事情的猜测。every time是连词，引导时间状语从句。that在句中不是连词，是指示代词，作operation的定语。)

[ 本帖最后由 john2005 于 2007-4-7 19:02 编辑 ]

thank you for posting and sharing these!
it is very helpful for english learner to improve english grammar.

Thank you ever so much.

Thank you ever so much.

thanks a lot

thanks a lot