一. it作人称代词
在特定的环境中，it可指人，如指婴儿、性别不详或身份不明的人。
e.g. 
1. The baby stopped crying as soon as it saw the woman.（指婴儿）
2. —Who is it?
—It’s me.（性别不详或身份不明）
二. itreported作形式主语或形式宾语
it可以作形式主语或形式宾语而把真正的主语或宾语后置。
1. 用it作形式主语的情况：
(1) It＋系动词＋形容词＋that从句
It is necessary/important/surprising/strange＋that从句，从句中谓语动词常用“should＋动词原形”形式，且should可省略。
(2) It＋系动词＋名词＋that从句
It is a pity/a shame/good news/a fact/an honour/a wonder/no wonder/our duty＋that从句。
e.g. It’s no wonder that you’ve achieved so much success.
难怪你取得了如此大的成功。
(3) It＋不及物动词＋that从句
It seems/happened/turned out /occurs/appears＋that从句。
e.g. It happened that he was out when I called on him.
当我去 拜访他的时候，碰巧他出去了。
(4) It＋系动词＋过去分词＋that从句
It is said /reported/ announced/ believed/ thought/ well known/ hoped/ pointed out/ found out/ suggested/ ordered/ advised/ requested/ insisted/ required/ demanded＋that从句。
e.g. It’s said that they have succeeded in carrying out this experiment.
据说他成功的进行了这次实验。
【注意】在“It is suggested/advised/ordered/requested/insisted/required/demanded＋that从句”结构中，从句要用虚拟语气，即谓语动词用“should＋动词原形”，should可省略。
e.g. It is suggested that you (should) spend more time in studying English.
建议你花更多时间学习英语。
(5) 其他句型，如It doesn’/It makes 等。
e.g. It doesn’t matter what you wear, as long as you look neat and tidy.
你穿什么不重要，只要看上去整洁就可以。
It makes no difference to me whether you go or not.
你去或不去对我来说都没有区别。
2. it作形式宾语的情况
(1)动词find, feel, think, consider, make, believe等后有宾语补足语时，则需要用it作形式宾语，把作为真正宾语的that从句后置。
e.g. We make it a rule that we get up at six every day.
我们每天6点起床已经成为了一个规则。
We all think it important that we should protect the environment from being polluted.
我们都认为保护环境免受污染很重要。
(2) 有些动词接宾语从句时，需在动词和从句间加it。这些动词有：hate, appreciate, take, owe, have, like, enjoy, make等。
e.g. I take it that you will agree with us.
(3) “动词＋介词＋it＋that”结构。常见的有see to, depend on, rely on等。
e.g. I will see to it that everything is ready on time.
我们会负责一切都按时准备好。
三. it可用在强调句型中
e.g. It was in the street that I met him yesterday.
我昨天是在街上遇见了他。
【注意】it作形式主语和it引导强调句的区别：it作形式主语代替主语从句，主要是为了平衡句子结构(特别是谓语较短时)，主语从句的连接词没有变化。
it引导的强调句用来对句中某一个成分加以强调，其结构为“It is/was＋被强调部分＋”。在正式英语中，无论强调什么部分，都要求用连词that，但是在一般情况下，如果被强调的
主语是“人”，也可用who/whom。值得注意的是，当被强调部分是状语时，只能用that，不要误用when，where，how等。强调句型中去掉It is/was和that后，句子仍然完整，而it作形式主语的句子则不能。
e.g. It is I who am setting off for London next week.
下周是我前往伦敦。
It was at six o’clock that we arrived at the mountain village.
我们是6点钟到达了这个山村。
It was at the hotel room that the gentleman had stayed.
这个绅士是呆在旅馆房间里。
四. it的特殊用法
1. 指代时间、距离、自然现象(天气)等。
e.g. It is half past two now. (指时间)
It is 6 miles to the nearest hospital. (指距离)
A lovely day, isn’t it? (指天气)
2. 指环境情况等。  
e.g. It is very noisy at the moment.　
3. 用于某些习语搭配。
make it　及时赶到；成功完成
catch it　受处罚，被责骂