中。灵活地运用it句型可使文章更高级，结构更紧凑。

　　(一)it作形式主语的常用句型

　　在英语中，为了避免头重脚轻，经常运用it作形式主语，而把真正的主语后置，作真正主语的常为动词不定式(短语)或从句，有时也可用动名词(短语)。

　　1．It takes (sb.) some time/energy/effort ...to do sth."做某事花费(某人的)时间/精力......"

　　It took them about half an hour to work out the difficult maths problem.

　　他们花了大约半个小时的时间才解出这道数学难题。

　　2．It is (was)＋形容词＋(of/for sb.) to do sth./It is (was)＋形容词＋that ...

　　①(2018·江苏高考书面表达)In that case, it is both convenient and economical to buy books according to the ratings.

　　在那种情况下，按消费排名购买图书既方便又经济。

　　②(2017·全国卷Ⅰ书面表达)It's necessary that you have a good knowledge of Tang Poetry.

　　你对唐诗有很好的了解是很有必要的。

　　3．It is (was)＋名词＋(for sb.) to do sth./It is (was)＋名词＋that ...

　　①It is a tradition and virtue for us Chinese people to help those in need.

　　帮助需要帮助的人是我们中华民族的传统和美德。

　　②(2017·全国卷Ⅰ书面表达)It's really a good idea that we should spend some time taking a brief look at the history of the Tang Dynasty.我们应该花点时间简要了解唐朝的历史，这的确是个好主意。

　　4．It is/was＋过去分词＋that ...

　　(2018·全国卷Ⅱ书面表达)It is believed that the English short film will be not only meaningful but also interesting.

　　相信这部英文短片不仅有意义，而且很有趣。

　　5．It seems/appears/happens＋that ..."似乎，碰巧......"

　　It seems that no one knows what has happened in the park.

　　似乎没有人知道公园里发生了什么事。

　　6．It is the first/second ...time that ..."这是第一/二......次......"

　　(2017·天津高考书面表达)It is the first time that I have been a volunteer, so I am now receiving all the training involved.这是我第一次做志愿者，所以我现在正接受所有相关的培训。

7．It is/has been＋一段时间＋since ..."自从......以来已经有多长时间了"