代替不定式，常用於下列結構：
It+ be+ adj./ n. +( for sb )to do sth;
It+ be+ adj. +of sb to do sth
It takes +sb+一段時間+ to do
①It is important for us to master a foreign language.對於我們來說，掌握一門外語是十分重要的。
②It is very kind of you to help me with my English.你幫我學英語，真是太好了。
③It will take about two days to drive to Shanghai.開車去上海大約要兩天的時間。
代替動名詞，常用於下列句型中：
It is no use doing sth. 做……沒有用。
It is no good doing sth. 做……沒好處
It is fun doing sth. 做……有趣
①It’s no use crying over spilt milk.覆水難收。
②It’s no good eating some fast food.吃快餐是沒有好處的。
③It’s great fun jumping into the swimming pool in summer.夏天跳進游泳池裏游泳是件有趣的事。
代替that從句，常用於下列句型中：
It be +adj./ n. +that從句
It be +過去分詞+ that 從句
It seems / appears / happens / turns out that 從句
It occurred to sb+ that從句
①I don’t know the restaurant, but it’s said that it’s quite a good one.
我不知道這家飯館，但聽說相當不錯。
②It is clear that he has made a mistake. 很明顯他犯了一個錯誤。
③It turns out that we’ll have a happy, peaceful festival.結果是我們又將度過一個歡樂祥和的節日。
④It occurred to me that I had left the key at home.我突然想到我把鑰匙落在家裏了。
二、It 作形式賓語
1.當賓語是動詞不定式、動名詞、賓語從句，且其後有賓語補足語時，爲了保持句子結構的平衡，通常用it作形式賓語，而將真正的賓語置於句末。常用it作形式賓語的動詞有：believe相信，find發現，feel感覺，think認爲，guess猜想，discover發現，notice注意到，make使……，讓……等。
①I don’t think it possible to master a foreign language without much memory work.我認爲要掌握一門外語而沒有大量的記憶是不可能的。
②I feel it hard to persuade my father to give up his plan.我覺得說服父親放棄計劃很難。
③He makes it a rule to keep a diary every day.寫日記成了他每天的習慣。
④Don’t take it for granted that all those who get good grades in the entrance examination will prove to be most successful. 不要認爲那些在入學考試中考得高分的`就是最成功的人。
2.某些及物動詞或短語不能直接加賓語從句，此時須用it作虛設賓語，然後接賓語從句，此種用法常用於like，dislike，hate， appreciate, depend on等後。
①I hate it when people talk with their mouths full.我討厭人說話時嘴裏含着食物。
②I like it when you kiss me.我喜歡你吻我的時候。