一、 it
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
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.
