关于从句中it与it is什么时候用哪种形式
A. 以下情况多用that.
(1) 如果先行词是all, much, anything, something, nothing, everything, little, none等不定代词，关系代词一般只用that，不用which。例如：
All the people that are present burst into tears.
Is there anything that I can do for you?
(2) 如果先行词被the only, the very, the same, the last, the best等词修饰，关系代词常用that(指人时，可以用who),例如：
This is the very book that I want to find.
That is the best film that we have seen.
I was the only person in my office that/who was invited.
(3) 当先行词被all, every, no, some, any, little, much所修饰时；
You can take any seat that is free.
(4) 当先行词被序数词或形容词最高级所修饰时。
It is the first foreign book that I have ever seen.
This is the most beautiful flower that I have seen.
(5) 在there be句型中，常用that.
There is a man that lives in that village.
(6) 先行词有两个，一个指人，一个指物，关系代词应该用that。例如：
The boy and the dog that are in the picture are very lovely.
B. 以下情况用which.
(1) 当关系词前面有介词时，指物常用which.
The prize for which he worked so hard was a new bike.
He built a telescope through which he could study the skies.
(2) 非限制性定语从句指物时，多用which.
Bamboo is hollow, which makes it light.
He wrote an article, which tells us about the computer.
(3) 如果先行词本身是that时，只用which.
This is that which he bought yesterday.
(4) which还有一种特殊用法，它可以引导从句修饰前面的整个主句，代替主句所表示的整体概念或部分概念。在这种从句中，which可以作主语，也可以作宾语或表语，多数情况下意思是与and this 相似，例如：
He succeeded in the competition, which made his parents very happy.
it是人称代词,作主语或者宾语.但是从句中关系代词作主语和宾语时就不能使用it了，定语从句中引导词可以是who ，which，where，whom，that，但不能用it
当形容词为 necessary, strange 等时，从句中的谓语动词应用虚拟语气 (should) do 形式。
若是其它普遍形容词，则从句为陈述句即可，但需遵循语法规则。
当不定式（短语）、动名词（短语）或从句在某个句子中作主语时,为保持句子结构前后平衡,避免头重脚轻,因此常用it作形式主语置于句首,而将真正的主语放在句尾.此时it只起先行引导作用,本身无词义.
e.g. It is wrong to tell a lie.
(说谎是错误的.) 〔It为to tell a lie的形式主语〕
It is no use arguing about it.
(争吵是没用的.) 〔It为arguing about it的形式主语〕
It is uncertain who will come.
(谁要来还不确定.) 〔It为who will come的形式主语〕
It 作形式主语的常见句型：
① It + be + 形容词 + to do sth. / doing / that ….
e.g. It is very important to learn a foreign language.
(学一门外语非常重要.)
It is useless crying over the spilt milk.
(覆水难收.)
It was really surprising that she married a man like that.
(她嫁给那样的男士真让人惊讶.)
② It + be +名词词组 + doing / that ….
e.g. It is no good telling lies.
(撒谎没好处.)
It is a pity that you didn’t go to see the film yesterday.
(你昨天没看成那部电影真遗憾.)
It is a truth that there would be no new China without the Communist Party.
(没有共产党就没有新中国,这是毋庸质疑的.)
③ It + be + 过去分词 + that ….
该句型常见动词有：say, hope, think, suppose, expect, report, know, believe, decide, etc.
e.g. It is said that they have invented a new type of computer.
(据说他们发明了一种新型电脑.)
It is believed that China will become one of the strongest countries in the world.
(大家都相信中国将会步入世界强国之列.)
It was reported that more than 170 thousand people died in the 2004 tsunami.
(据报道,至少有十七万人在2004年那场海啸中丧生.)
④ It + seems / appears / happens等不及物动词 + that ….
e.g. It seems that he enjoys pop songs very much.
(似乎他非常喜欢流行歌曲.)
It appears that Tom might change his mind.
(看来汤姆可能会改变主意.)
⑤ 若句子是疑问形式,就只能用it作形式主语.
e.g. Does it matter much that they won’t come tomorrow?
(他们明天不来很重要吗?)
Is it true that he will go abroad next week?
(他下周出国是真的吗?)
⑥ It + takes + (sb.) + some time + to do sth.
这是一个表示“（某人）花多少时间干某事”的句型,其中it是形式主语,代替后面的不定式（to do sth.）,句型中的sb.也可以省略.
e.g. It took me some time to read the reading materials.
(我花了一些时间才读完那段阅读材料.)
It took him fourteen hours to go to New Zealand from Shanghai by plane.
(从上海乘飞机去新西兰花了他14小时.)
How long does it take you to go to Beijing from Qingdao by train?
(从青岛坐火车到北京一般要花多久时间?)
I am not sure, but I think it takes at least nine hours to get there.
(我不能肯定,但估计至少要九个钟头才能到那儿.)
二、It 用作形式宾语
当不定式（短语）、动名词（短语）或从句在某个句子中作宾语时,为保持句子结构平衡,避免句式结构的混乱,常用it作形式宾语,而将真正的宾语放在句尾.此时it仍只起先行引导作用,本身无词义.
下列四种情况须用it 作形式宾语：
① 当不定式（短语）、动名词（短语）或从句在复合宾语结构中作某些动词的宾语时（如think, make, find, consider, feel, suppose等）；
e.g. They found it pleasant that they worked with us Chinese.
（他们发现与我们中国人一起工作很愉快.）
I don’t feel it difficult to understand the Special English.
（我觉得理解英语特别节目并不难.）
He makes it a rule never to borrow money.
（他立志决不向别人借钱.）
I think it no need talking about it with them.
（我认为没必要跟他们谈.）
② 某些表示“喜、怒、哀、乐”的动词,如like, enjoy, love, hate等,往往不能直接接宾语从句；
e.g. I don’t like it that he’s so lazy.
（我不喜欢他那么懒惰.）
I hate it when my mother asks me to eat eggs.
（我讨厌母亲要我吃鸡蛋.）
③ that引导的宾语从句不能直接作介词的宾语；
e.g. You may depend on it that we shall always help you.
（尽管放心,我们会随时帮你的.）
Would you see to it that she gets home early?
（你负责保证她早到家,好吗?）
He insisted on it that he was innocent.
（他坚持说自己是无辜的.）
④ 由及物动词与介词组成的固定搭配中,宾语从句若作该动词的宾语时,须借用it.
e.g. I leave it to your own judgement whether you should do it.
（我让你自己判断这事是否该做.）
We owe it to you that there wasn’t a serious accident.
（多亏了你才没有发生严重事故.）
it is neccessary for us to
i think it的it是形式宾语 形式主语和形式宾语通常用在不定式中,用it代替.
形式主语it +谓语或宾语+ adj +to do sth
如It is useful to study English.在这个句中真正的主语是后面的to do形式,it 叫形式主语.
形式宾语的句子 主语 谓语或宾语 it adj to do sth
如I think it is intresesting to dance .真正的宾语是to do形式,it叫形式宾语.
形式主语和宾语的使用都是为了简化语句,让别人一目了然.之所以用it 代替是 避免头重脚轻.
“it”作形式宾语时的几种特殊结构
在英语中,当动词不定式、动名词或从句在句中充当宾语且其后又带有宾语补足语时,习惯上要在宾语补足语前使用一个没有具体意义的形式宾语it,而将真正的宾语移到补足语的后面,构成“动词＋it＋宾语补足语＋不定式（动名词或从句）”结构.如：
She felt it her duty to take good care of them.
她感到好好照顾他们是她的责任.
I think it no good going there now.
我认为现在去那里没有好处.
The headmaster has made it clear that the school meeting will not be put off.
校长说得很清楚校会不会推迟.
但我们常会碰到it用作形式宾语时之后没有形容词或名词作宾语补足语的情况,这一点在近几年高考试题中经常涉及,需引起同学们的注意.
I hate_____ when people talk with their mouths full. (NMET 1998)
A. it B. that C. these D. them (答案为: A)
我讨厌人们一边吃饭一边说话.
I like _____ in the autumn when the weather is clear and bright. (全国卷 2004)
A. this B. that C. it D. one (答案为: C)
我喜欢秋天晴朗的天气.
以上这两道高考题中,it用作形式宾语,它的真正的宾语则是由when引导的从句.“it”的这种特殊用法常出现在以下几种结构中.
A. 动词＋it＋when / if 从句.常见于appreciate, enjoy, like, dislike, love, hate, prefer等少数动词之后.
We would much appreciate it if you could do us that favour.
要是你们能帮我们这个忙,我们将非常感激.
I'd prefer it if I didn't have to finish the work.
要是我不必完成那项工作,那就太好了.
B. 动词+ it + that从句.常见于like, take, have, put等少数动词之后（it相当于that从句的同位语）.
I take it that they will succeed sooner or later.
我认为他们迟早会成功的.
The newspapers have it that some Japanese firms have gone bankrupt.
报上说一些日本商号破产了.
Rumor has it that Mary is getting married. 谣传玛丽快结婚了.
I take it that you have been out. 我以为你一直出门在外.
Let me come and stay. You can put it that it was arranged before.
就让我来呆在这儿吧,你就说原先就是这样安排的.
C. 动词 + it + 介词短语 + that从句.常见于bring it to one's attention;
owe it to sb; take it for granted 等结构中.
I owe it to you that I finished my work in time.
多亏有你我才及时完成了工作.
Don't take it for granted that they will support you.
不要想当然地认为他们会支持你.
D. 动词 + 介词(短语) + it + that从句.常见于answer for, depend on, rely on, insist on, look to, see to, stick to 等结构中.
I'll see to it that everything is ready in time.
我将负责按时做好一切准备.
