It用法及其句型和固定搭配，是高中英语语法的重点、难点，又是高考英语的热点，因此应给予充分的重视，现将it用法归纳如下：

一、作人称代词

1、it代替前面（或后面）的单数名词或分句等所表示的事物。

e.g. You cannot eat your cake but have it.（it代替前面的cake）Although we cannot see it, there is air all around us. （it代替后面的air）They say he has left town, but I don’t believe it. （it代替前面They…town分句中的情况）

2、代替有生命但不能或不必分阴阳性的东西（包括婴儿）。

e.g. Yesterday we saw a big tree. It was fully twenty metres high. （it代替前面的tree）The baby cried because it was hungry. （it代替前面的baby）

3、在某些习惯说法中，可以代替人。

e.g. -- Someone is knocking at the door, Peter. -- Who is it? -- It’s me.

-- Who are singing? -- It is the children.

-- The light is still on in the lab. It must be the third-year students doing the experiment.

4、it与one的区别：这两个词都可以代表前面说过的名词，但it用于同名同物的场合；one则用于同名异物的场合。

e.g. -- Do you still have the bicycle? -- No, I have sold it.

-- Is this knife yours? -- No. It is Xiao Zhang’s. Mine is the one on the desk.

5、it与that的区别：两词都可代替某一特定名词，但that指同一类，并非同一个。

e.g. The climate of South China is mild（温和的）; I like it very much.（it指the climate of South China）The climate of South China is much better than that of Japan.（that指the climate）