在學習代詞it在句中作主語時，需要注意以下幾點：
一、 it作形式主語時，除了教科書上講解的可代替不定式和主語從句外，還可代替動名詞。例如：
It’s no use crying over spilt milk. 后悔無用。
It’s no good taking in much fat and sugar. 攝取大量的糖和脂肪沒有好處。
另外，學習代詞it作形式主語時，還要注意學會使用下列常用句式。
It takes / took / will take sb. some time to do sth. 做某事花費某人多長時間
It looks / looked as if ... 看起來好像……
It seems / seemed that ... 似乎……
It (so) happened that ... 碰巧……
It turns / turned out that... 原來是……
It is said / reported / believed / hoped / well known / suggested ... that...
據(jù)說 / 據(jù)報道 / 相信 / 希望 / 眾所周知 / 建議……
It occurred to sb. that ... 某人想起……
It was / will be + 一段時間 + before ... 過 / 要過多久才……
It is / was / has been / had been + 一段時間 + since ... 自從……已經(jīng)……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、非人稱代詞it，還可指代this，that。例如：
-Do you know what this is in English? 你知道這個英語怎么說嗎?
-It’s a cell phone. 這是手機。
-How about that? 那個呢?
-It’s a computer. 那是電腦。
三、指代有生命但不能或不必區(qū)分性別的人或物。例如：
Yesterday we saw a big elephant in the zoo. It is about three meters high. 昨天我在動物園看到一頭大象，大約三米高。(it指代elephant)
Listen! The baby is crying out loud! It must be very hungry. 聽!那嬰兒在大聲哭!一定是餓極了。(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，上學路上把我最喜歡的手表給弄丟了。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.母親生日那天我給她買了禮物--一條昂貴的項鏈。(it指代上文中的present)
Although we cannot see it, there is air all around us. 盡管我們看不到空氣，但我們周圍到處都是空氣。(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? 李老師已出國深造。你聽說(這件事)了嗎?
I have told him many times not to be late for class, but it didn’t help. 我已經(jīng)告訴他好多次上課不要遲到了，但就是沒有用。
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.
