1、在主语是不定式或者从句的时侯，为了避免句子看上去头重脚轻，我们可以使用it来做形式主语，平衡这种失调的感觉。比如：
(1) to finish our work in 5 days is a difficult task.
可以变成：it is a difficult task to finish our work in 5 days.
(2)that he won the game is impossible.
可以变成：it is impossible that he won the game.
(3)i thought it better to work carefully.（此时，it作形式宾语）
2、当我们希望强调句子的主语、宾语、状语等成分的时侯，我们可以使用it引导的强调句。
(1)you are wrong.
可以变成：it is you who are rong.
(2)we drove japanese troops out of china in 1945.
可以变成：it was in 1945 that we drove japanese troops out of china.