在学习代词it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’s no use crying over spilt milk. 后悔无用。

It’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 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 seems / seemed that ... 似乎……

It (so) happened that ... 碰巧……

It turns / turned out that... 原来是……

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 occurred to sb. that ... 某人想起……

It was / will be + 一段时间 + before ... 过 / 要过多久才……

It is / was / has been / had been + 一段时间 + since ... 自从……已经……例如：

It will take us a very long time to improve our reading.

It looks as if it is going to rain.

It seems that something unusual will happen to them.

It (so) happened that I had no money with me.

It turns out that he is a famous computer expert.

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 me that I should have a talk with John.

It will be a long time before we meet again.

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’s a cell phone. 这是手机。

-How about that? 那个呢?

-It’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’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’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?

-It may be our director.

-Who is it?

-It’s me.