作形式主语，而将真正的主语(动词不定式短语、动名词短语、名词性从句等)放在句末。这是一种习惯表达法，常见的句型有：

　　(1)It＋is/was＋形容词＋(for sb.)＋动词不定式短语。

　　It is necessary for older people to understand what they think and feel.

　　年长的人有必要了解他们在想些什么，他们有什么感觉。

　　(2)It＋is/was＋形容词＋of sb.＋动词不定式短语。

　　It's clever of you to do like this.你这样做很聪明。

　　(3)It＋系动词(be动词除外)＋形容词＋动词不定式短语。

　　It feels strange to have a twin sister.

　　有个孪生姐妹感觉很奇怪。

　　(4)It＋will be/is/was＋形容词/名词＋动名词短语。

　　It's a waste of time doing the work.

　　做这项工作是在浪费时间。

　　It's no good waiting.等候是没有用的。

　　(5)It＋is/was＋形容词＋that从句。

　　It is certain that he will attend the meeting.

　　他一定会参加会议的。

　　It is important that we (should) study hard.

　　我们应该努力学习，这是很重要的。

　　注意　It is clear (plain，true，certain，sure，obvious，certain...)＋that­clause(从句用陈述语气)

　　It is important (necessary...)＋that­clause(从句用虚拟语气，should可省略)

　　(6)It＋is/was＋one's turn (duty，pleasure，a pity，great fun，a habit，time，a rule，a shame，no wonder)＋to do sth.。

　　It's your turn to be on duty tomorrow.明天轮到你值日了。

　　It must be great fun to fly to the moon in a spaceship.

　　乘宇宙飞船飞往月球一定很有趣。

　　(7)It is/was＋名词或名词短语＋从句。

　　It's a wonder that you could finish such a hard task in such a short time.

　　真奇怪，你竟然在如此短的时间内完成如此艰巨的任务。

　　注意　"It is a suggestion (my wish)＋that从句"句型中，从句中谓语部分用should＋动词原形，should可省略。

　　It's his suggestion that we should go by train.

　　他建议我们乘火车去。

　　(8)It is＋过去分词＋that从句。

　　It is reported that there will be a concert this Saturday night.据报道本周六晚有一场音乐会。

　　It is suggested that some measures (should) be taken to stop pollution.据建议要采取一些措施来防止污染。

　　注意　该句型常为：It is said/hoped/thought/supposed/known/reported/announced/decided/

suggested/believed...that...等，该句型常可以转换成含有宾语从句的复合句