在英语学习中，代词 it并非看似的那么简单！它的用法值得我们好好探讨一下，当它在句中作主语时，需要注意以下几点：
一、 it作形式主语时，除了教科书上讲解的可代替不定式和主语从句外，还可代替动名词。例如：
It&# 39; s no use crying over spilt milk. 后悔无用。
It&# 39; s no good taking in much fat and sugar. 摄取大量的糖和脂肪没有好处。
另外，学习代词 it作形式主语时，还要注意学会使用下列常用句式。
It will take us a very long time to improve our reading.
It takes/ took/ will take sb. some time to do sth. 做某事花费某人多长时间
It looks/ looked as if... 看起来好像……
It looks as if it is going to rain.
It seems/ seemed that... 似乎……
It seems that something unusual will happen to them.
It (so) happened that... 碰巧……
It (so) happened that I had no money with me.
It turns/ turned out that... 原来是……
It turns out that he is a famous computer expert.
It is said/ reported/ believed/ hoped/ well known/ suggested that... 据说/据报道/相信/希望/众所周知/建议……
It is said that they have got married.
It is well known that the bird flu is a very serious disease.
It is hoped that every one of us will protect wild animals.
It occurred to sb. that... 某人想起……
It occurred to me that I should have a talk with John.
It was/ will be+一段时间+ before... 过/要过多久才……
It will be a long time before we meet again.
It is/ was/ has been/ had been+一段时间+ since... 自从……已经……
It is two years since he went abroad.
二、非人称代词 it，还可指代 this/ that。例如：
— Do you know what this is in English? 你知道这个英语怎么说吗？
— It&# 39; s a cell phone. 这是手机。
— How about that? 那个呢？
— It&# 39; s a computer. 那是电脑。
三、指代有生命但不能或不必区分性别的人或物。例如：
Yesterday we saw a big elephant in the zoo. It is about three meters high. 昨天我在动物园看到一头大象，大约三米高。（ it指代 elephant）
Listen! The baby is crying out loud! It must be very hungry. 听！那婴儿在大声哭！一定是饿极了。（ it指代 baby）
四、指代上下文中所提到的事物。例如：
Unfortunately, I lost my favorite watch on my way to school. It cost me more than 200 yuan. 真不幸，上学路上把我最喜欢的手表给弄丢了。它花了我 200多元。（ it指代上文中的 watch）
On my mother&# 39; s birthday I bought a present for her. It was an expensive necklace. 母亲生日那天我给她买了礼物--一条昂贵的项链。（ it指代上文中的 present）
Although we cannot see it, there is air all around us. 尽管我们看不到空气，但我们周围到处都是空气。（ it指代下文中的 air）
五、指代上文中提到的事件。例如：
Mr. Li has gone abroad for further study. Have you heard of it? 李老师已出国深造。你听说（这件事）了吗？
I have told him many times not to be late for class, but it didn&# 39; t help. 我已经告诉他好多次上课不要迟到了，但就是没有用。
六、指代身份不明的人。例如：
— Look! Something is coming. Who can it be? 看！有人来了。会是谁呢？
— It may be our director. 应该是我们的主任。
— Who is it? 是谁？
— It&# 39; s me. 是我。
