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．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.
