１．作人称代词
John likes playing Pingpong．/ He always does it in the afternoon．(指代上下文提到的事物)；/ It's time we went home. / How far is it from here to your home ? / It is getting warmer and warmer．/ It's very quiet at the moment．(可指时间、天气、环境等)
２．引导词
Ａ．作形式主语，代替由不定式、动名词或从句表示的真正主语。
It's important for us to learn a second language．/ It's no use talking to him．/ It's known to all that the earth goes round the sun．
