it
1.it往往用作主语或宾语，预示后面的不定式，如It is my duty to help him和I consider it my duty to help him。it有时用作主语或宾语，预示后面的动名词，但不及预示不定式的普通，如It is my duty helping him和I consider it my duty helping him。那动名词前面往往有my，your，this，that等词，如It is no good my telling him so和I consider it no good, this telling him so。注意this前面有逗号。下面的两句里it is有时省去：
- What a bore it is to be kept waiting for an hour!
- What a bore it is being kept waiting for an hour!
2.it 有时用作主语，预示后面的一个或多个名词。例如：It is very curious, that story. 和It is known to none of us, his name and nationality. 这两句里的逗号可以省去。
3.it往往用作主语或宾语，预示后面的以that引导的从句，如It is true that he is a foreigner和I consider it true that he is a foreigner。那that有时省去。
4.it有时用作主语或宾语，预示后面的一个以how，what，when，where，whether，why等词引导的从句，如It is not known how he came here和I want to make it clear whether she is related to him or not。比较下面两句：
- It is not known what he said yesterday. （不知道他昨天说了什么话。）
- It is not true what he said yesterday. （他昨天说的话是不确实的。）
第二句里的what he said yesterday不是疑问句，等于2所讲的名词。
5.3里所举的It is true that he is a foreigner里的true是形容词，不是副词。下面三句里It is和that中间的不是形容词而是副词、状语短语和状语从句：
- It is there that accidents often occur.
- It is through carelessness that accidents often occur.
- It is when the lights have gone out that accidents often occur.
这三句是下面三句的强势式：
- Accidents often occur there.
- Accidents often occur through carelessness.
- Accidents often occur after the lights have gone out.
Although the work progresses, it is but slowly里用slowly而不用slow，因为it is but slowly等于it is but slowly that the work progresses。
6.下面两句意思不同：
- It is help that I need. （我所需要的是帮助［不是其他东西］。）
- It is I that need help. （需要帮助的就是我［不是别人］。）
这两句语气都比I need help强。
就语法讲，that是关系代词，它的先行项是It，所以在第二句里在理论上该用needs而不用need，但习惯上那动词的人称是依照那表语（例如第二句里的I）而不依照那先行项It的。
第一句里的that改作which和第二句里的that改作who都可以，但都不及用that普通。
7.5和6里所讲的都是强势式。两种句法似乎相同而实际不同：5里所举的句子里的that是连接词，引导用作It的同位语的名词从句，那主句里的动词（例如is）后面接副词、状语短语或状语从句。6里所举的句子里的that是关系代词，引导修饰It的形容词从句，那主句里的动词（例如is）后面接名词或代词。
8.下面每句里的it和用作它的同位语的名词从句贴近，却不可把it省去：
- I take it that you are about twenty-five.
- He will have it that he has always done his best.
- I will answer for it that my brother is honest.
- You may depend (up) on it that he will be persuaded before long.
- He insisted (up) on it that he was innocent.
- I will see to it that everything is ready by five o'clock.
- You may rely (up) on it that things will come right.
- Can you swear to it that you saw him enter the room this morning?
- There is no doubt about it that the weather is perfect today.
- I took his word for it that he would make another effort.
第五句里的 (up) on it，第六句里的to it，第八句里的to it和第九句里的about it都可以省去。
9.A teacher's duty is to teach和It is a teacher's duty to teach意思相同，但语法结构不同。下面第一句里的whose duty is to teach和第二句里的whose duty it is to teach在语法结构上的不同相当于上面两句的不同：
- A teacher is a person whose duty is to teach.
- A teacher is a person whose duty it is to teach.
10.可以说give it me，send it him，bring it us等，但通常不说give me it，send him it，bring us it等。在美国通常说give it to me，send it to him，bring it to us等。
11.下面各句里的it代表前半句所说的情况：
- He is thirty years old, and looks it.
- You are much stronger now, though you may not look it.
- She is a nurse, and she looks it.
12.下面各句是爱尔兰语，不是正当的英语：
- It is very happy that he is. (=He is very happy.)
- It is pleased I am to see you home again. (=I am pleased to see you home again.)
- Is it writing a letter you are? (=Are you writing a letter?)
