It是英语中高频词之一，几乎无处不见。在用法上，能够作人称代词，非人称代词，先行词，用于强调句型，以及习惯用法。
22.1 人称代词it (Personal it)
It作人称代词，其功能是替换，代替一个词，词组或整个句子，避免重复。it可指事物，也可以指人。
指代婴儿或较年幼的小孩，或者用于某些句型中，指代人。
22.1.1 指代事物
--Look at that bird. It always comes to my windows.
--The sun kept changing everything as it moved across the sky.
--She made some soup and gave it to the children.
22.1.2 指代人
--Her new baby is tiny. It only weighs 2 kilos.
--Ann (on phone): Who is it?
--Bill: It's me.
-Is that Tom over there?
-No, it's Peter.
22.1.3 指代已经叙述的一个分句或一个句子，也可指隐含的整个情况或事实。
--A: Who said that I was crazy?
--B: I said it. ("it" means "that you were crazy")
--Rome was sacked by the Visigoths in 410 A.D. It (the sack of Rome)
--was the end of civilization as the West had known it. 道歉的句子给爱人
--He has gone out of his way to help us. We won't forget it.
--It doesn't matter.
--It's my fault.
然而，it代替整个句子的用法，不能用在曾国藩书法know, remember, try, tell,forget等动词之后。例如：
--Remember what he told you.
-I'll remember.
(不能说: I'll remember it.)
22.1.4 it用在部分介词或动词后面，构成习惯用法，主要用于非正式文体或口语当中，此时it本身意义
含糊。
--At last we've made it. 喷气式飞机（成功）
清明放假安排 --You're in for it. （将陷入困境/室外效果图麻烦）
--have a hard time of it （感到生活困难）
--make a go of it （取得某方面的成功）
--stick it out （坚持） 儿童图片
--We'll have it out with them some day. (决一雄雌；讲个明白)
--You can't get away with it. （做错事而不被发觉/处分）
--Then we decided to go swimming at midnight just for the hell of it. (只是为了好玩）
--The last train's gone. Come on, we'll have to foot it. （走着去）
--If you go camping, you'll have to rough it. （忍受辛苦）廉洁从业承诺书
--If the teacher es you doing that, you'll catch it. （出麻烦）
西宁特色美食 --I'm afraid we've had it; the bus left five minutes ago. (糟糕了；倒霉了）
--If anyone catches him damaging tho trees, he'll be for it. (为此受处罚)
--When the thieves saw the policeman coming, they ran for it. (逃之夭夭）
--With clenched fists and t faces, the two boys went at it until they were exhausted.
