It 最基本的用法是人称代词，主要代表刚提到的东西以避免重复。

Look at that car. It’s going much too fast. Where is the car key? ---- I put it in your drawer.

也可以代表抽象的东西。

It’s all my fault .

也可以指动物或未知性别的婴儿。

What a beautiful baby， is it a boy? How about the baby? ---- I’ll take care of it.

还可代表一个彼此都知其何所指的东西。

Where does it hurt? If he doesn’t come, I can’t help it. You have saved my life. I shall never forget it.