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）
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）
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.
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.
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）