1. 形式主语
为了保持句子平衡，通常将主语从句移至句末而在原主语位置使用形式主语it。具体说来，有以下两种情况：对于以that引导的主语从句，通常用形式主语代主语从句；对于以连接代词(副词)引导的主语从句，可以使用形式主语代主语从句，也可直接在句首使用主语从句。如：
It is important to study English.学习英语重要。
It may be said that the measure of the worth of any institution is its effect in enlarging and improving experience.
据说，任何记工员的价值衡量是其在扩大和丰富经验的效应。
It is a pity that many parents don’t realize the importance of setting a good example.
很多父母并未意识到树立良好榜样的重要性，这令人遗憾。
It is a problem facing students whether they can resist the temptation of net games.
能否抵御网络游戏的诱惑是摆在中学生面前的一道难题。
It’s a shame (that) he didn’t come. 很遗憾他没来。
Whether it will do us harm remains to be seen.= It remains to be seen whether it will do us harm. 是否对我们有害还要看一看。
Where I spend my summer is no business of yours.= It is no business of yours where I spend my summer. 我在哪里过暑假不关你的事。
It + is/was + adj + for/of sb to do sth.
注意：该句型中，如果形容词含有褒义或贬义，那么，后面的介词要用of；除此之外的情况用for，意为“对…来说”。
(1) It's difficult for them to get here much before seven.
他们很难在7点以前早早地来到这里。
(2) It was most kind of you to meet me.
你来接我真是太好了。
It takes / took / will take + sb＋时间段＋ to do sth 某人做某事花了…时间。
注意：该句型中，动词后的人称代词要用“宾格”，后面要用动词不定式短语！
(1) It took her three hours to repair her bike.
她花了三个小时修理自行车。
(2) It takes me an hour to walk there and back.
我步行往返要花一个小时。
It is up to sb to do sth. 由某人决定做…。
该句型中的动词不定式短语有时可以省略。
(1) It is up to you. 由你决定。
(2) It is up to him to make the final choice.
该由他来做最后的选择了！
It is up to the regulating authority to put the measures into effect.
应该由管理部门落实这些措施。
It looks/seems/happens that (as if)…：表“似乎… ”。
(1) It seems that you are letting things get you down.
看上去你快要累趴下了。
It seems that the fifth man is one John Cairncross.
第五位好像是一个名叫约翰·凯恩克罗斯的人。
It seems that he is not quite himself today.
他今天好像不怎么舒服。
It seems that your proposals are good for his study.
似乎你的建议对他的学习有帮助。
(2) It seems to me that she will not come to the birthday party.
依我看，她不会来参加派对。
It seems to me your wife was a shallow woman and you're well rid of her.
在我看来你妻子是一个浅薄的女人，幸好你离开了她。
It seems to me that this crime was foreseeable and this death preventable.
在我看来这起罪行是可以预见的，死亡也是本可避免的。
(3) It seems as if he did not see me.
似乎他没有看见我。
It seems as if the blanket of depression is in some way necessary to help them blot out the even greater pain of real life
似乎这种弥漫的抑郁情绪多少能帮助他们忘却一些现实生活中更大的痛苦。
注意：
(1) It seems that…：从句用陈述语气。
(2) It seems as if …：从句用虚拟语气。
It is
said/believed/reported/hoped/ well-known that从句：表示“据说/人们相信/据报道/人们希望/众所周知/建议…”。
(1) It's said that he has already made the final decision.
据说，他已经做出了最后的决定。
It turned/turns out that…：结果是…
(1) It turned out that he was innocent.
结果表明，他是无辜的。
It's a great honor to be invited. 被邀请是一件很荣幸的事。（真正主语：to be invited）
It is no use crying over spilt milk. 因牛奶溅出去了而哭是没有用的。
（真正的主语：crying over spilt milk）
It falls to you to decide where to go for holiday.
去哪里度假还是由你来做决定吧。
It falls to me to finish the work.
完成这项工作的任务落在我头上。
It falls to you to do the report today.
今天该你负责做报告。
It fell to me to get rid of them.
由我来负责摆脱他们。
It fell to me to tell her the news.
我负责把这个消息通知给她。
2. 形式宾语
当宾语从句后跟有宾语补足语时，通常在宾语从句处使用形式宾语 it，而将真正的宾语从句移至句末。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。it 作形式宾语多用于下列句型中。如：
He didn’t make it clear why he had left so early. 他没有讲清他为什么那么早离开。
He doesn’t want it to be known that he’s going away. 他不想让人知道他要离开。
We think it important to learn English. 我们认为学习英语很重要。（真正的宾语：to learn English）
I found it pleasant walking in the park. 我发现在公园里走着是件乐事。（真正的宾语：walking in the park）
I find it beneficial to read more literature works. 我发现多阅读一些文学作品很有益处。
I find it very difficult to learn English well. 我发现学好英语很难。
She thinks it her duty to teach students well. 她认为教好孩子们是她的职责。
I think it unlikely that they would approve of this proposal.
我认为他们不大可能赞成这项建议。
具体如下：
1）、主语+think+形式宾语it+形容词/名词+to do sth.
I think it very important to learn English well.
我认为学好英语非常重要。
He thought it easy to solve the problem.
他以为解决那个问题很容易。
2）、主语+believe+形式宾语it+形容词/名词+to do sth.
Mike believes it difficult to fix the computer.
迈克认为维修那台计算机很难。
I believe it a virtue to help others.
我相信帮助别人是一种美德。
3）、主语+make+形式宾语it+形容词/名词+to do sth.
A good teacher makes it a pleasure to learn.
好的老师能够寓教于乐。
The telephone makes it possible for us to communicate at all times and all places.
电话使得我们随时随地交流变为可能。
4）、主语+find+形式宾语it+形容词/名词+to do sth.
She found it hard to ask Mary to give her a hand.
她发现让玛丽帮她一个忙很难。
Those kids find it interesting to learn English.
那些孩子发现学英语很有趣。
5）、主语+consider+形式宾语it+形容词/名词+to do sth.
We consider it an honor to work with you here.
我们认为和你一起在这里工作是一种荣幸。
I consider it necessary to let kids get into good eating habits from childhood.
我认为让孩子从小养成良好的饮食习惯很有必要。
6）、主语+feel+形式宾语it+形容词/名词+to do sth.
The doctor felt it necessary for you to give up smoking as soon as possible.
医生认为你尽快戒烟很有必要。
I feel it our duty to help our parents do the housework.
我觉得帮助父母做家务是我们的义务。
7）、 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）。例如：
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。
8）、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。例如
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。
9）、 that引导的宾语从句不能直接作介词的宾语。例如：
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。
10）、 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。例如：
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。