一、it用来替代前面提到的同一事物时,被替代的名词可以是可数名词,也可以是不可数名词.例如: ①We are studying in a school with trees all around it.我们在一个周围都是树的学校里学习。(it替代可数名词school) ②—Have you still kept your old furniture?—No, I have sold it.——你还留着你的旧家具吗？——不，我已经卖了(it替代不可数名词furniture) 

二、在英语中，由于句法结构的需要，常用it作形式主语或宾语，而把真正的主语或者宾语后置。
1．形式主语：不定式、动名词以及主语从句作主语时，为避免“头重脚轻”，要将真正的主语后置。例如：
①It seemed impossible for him  to get back home. 他似乎不可能回家。（不定式作主语）
②It was worthwhile searching for his "roots." 寻找他的”根”是值得的。（动名词作主语）
③It was obvious that his ancestor taught his younger generation little of his own language. 很明显他的祖先很少教年轻一代母语。（主语从句作主语）
常用的句型：
1) It is / was difficult (easy, hard, important, necessary, useful, fit, possible, a pity, a Pleasure…) (for sb.) to do sth. 
2) It is / was kind (nice, wise, clever…) (of sb.) to do sth.= Sb. is / was angry ( ashamed, delighted, disappointed, excited, glad, happy, pleased, proud, sorry, surprised, thankful…) to do sth. 
3) It is better (interesting, tiring, enjoyable, funny, fun, a bore, a waste of time…) doing sth.
4) It is clear (plain, true, certain, sure, obvious…) + that clause + 陈述语气
5) It is important ( necessary, strange, a pity…) + that clause + (should ) do
6) It is a suggestion (my wish, 及与disco动词有关的名词) + that clause +(should) do
7) It is ordered (及与disco动词有关的动词的被动语态) + that clause +(should) do
8) It is high / very time that we should have lunch / had lunch here. 
    It is the first / last time that we have had lunch here (just).
    It is the third time that we shall have lunch here (in a moment).
2. 形式宾语：当不定式、动名词、that 从句作宾语，又有自己的宾语补语时，要用it 作形式宾语，而把真正的宾语后置。例如：
①Marx found it important to study the situation in Russia.  马克思发现研究俄国的形势很重要。
②The fisherman made it a rule that he never cast his net more than four times a day.渔夫有个规矩，他一天不能撒网超过四次。
③The ancient people took it for granted that the earth was flat. 古人理所当然地认为地球是平的。
④We think it necessary reading aloud every morning to improve our English.我们认为有必要每天早上大声朗读以提高我们的英语。
能够用于形式宾语句型的动词有think, make, find, consider, feel, take等。

三、it可以替代前面的某一名词或整个句子的内容。这时，可用that替换，也可以改为非限制性定语从句。有时it可以前置替代，用以代替后面整个句子的内容。这时，不可用that替换。
例如: ①He said he had never met her,and it is not true.他说他从没见过她，这不是真的。(it替代前面整个句子He said...) ②I hate it when people talk with their mouths full.我讨厌人们嘴里塞满东西说话。(it代替后面整个句子when people talk with...)。