it用作人称代词和非人称代词的用法总结
1. 作人称代词
人称代词it一词是有实际意义的，是单数第三人称代词。主格与宾格相同。
（1）指代人以外的一切生物或事物，如前文提到的事物或未指明但谈话双方都知道的那件事。
Qingdao is a beautiful city, isn’t it?
It doesn’t matter.
（2）替代前文中的this，that
This is your plan? Will you hand it in?
（3）指代婴儿，小孩或性别不明的人
The baby is crying. It might be hungry.
----Someone is ringing. Who might it be?

----It must be my friend Tom. He (不可用it) wants to see you.

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2. 作非人称代词
it用作无人称的主语，不指物，而表示时间、天气、环境、气候、季节、距离、金钱等自然现象，或指事物的状态。
It was very noisy at that time.(指代环境)
It is half past three now.(指代时间)
It is six miles to the nearest hospital from here.(指代距离)
It was very cold.(指代天气)
It is 5 dollars.(指代金钱)
3. it, one, that, those和which的区别：
1) it 指上文提到过的同一样事物，one则泛指上文提到过的同一类事物中的一个；
There are many dictionaries in that bookstore. So I bought one.(one泛指其中之一)
2)that 作代词替代前面提到的事物(多用于比较)，可数名词，不可数名词皆可，代替可数名词可用the one代替，that的复数形式为those,只可替代可数名词的复数，在句中相当于the ones.
The head teacher in your class is younger and more active than that in theirs.
The students who do best in the exam are not always those with the best brains.
3)it和which都可用来指代某一事件，关键要看两个句子之间是否有并列连词，如果有就用it，反之用which。
He was badly ill, and it made his mother worried.
He was badly ill, which made his mother worried.(which指代前面整个主句，在定语从句中作主语)

（1）It's time to do... 到了干……的时间了。
（2）It's time for sth. 到了……的时间了。
（3）It's time (for sb.) to do sth. 
到了某人干……的时间了。
（4）It's (high/about) time＋that 从句 
到了某人干……的时间了。
（5）It/This is the first/second...time that...
这是第一/二……次……
提示：①“It‘s (high/about) time＋that 从句”中，从句的谓语动词用一般过去时或 “should＋动词原形”，且 should不能省略。
②“It/This is the first/second...time＋that从句”中，从句的谓语动词要用完成时态，当句型为 It is the first time that时，从句谓语动词用现在完成时，当句型为 It was the first time that 时，从句谓语动词用过去完成时态。
① It‘s time for lunch/to have lunch.
到吃午饭的时间了。
② It’s time that the kids were/should be in bed.
到了孩子们睡觉的时间了。
③ This is the first time that I have been to the Great Wall. 这是我第一次参观长城。

1．________you pay a visit to Shanghai，I’ll show you around.
A．Next time        B．Next time when
C．Last time         D．Last time when
2．The police set about arresting the suspect________they found enough evidence to prove him guilty.
A．immediate        B．as long as
C．the moment       D．as far as
3．I hope that my father will quit smoking because I want him to live________my grandpa does.
A．as a long and healthy life as
B．as long and healthy a life as
C．such long and healthy a life as
D．so long and healthy a life as
4．As a teacher，I seldom give my students________as they can not work out.
A．so difficult a problem
B．a so difficult problem
C．such difficult a problem
D．too difficult a problem
5．In my opinion，all Mr White________good to his students in his class at present.He is very strict in their study.
A．does does does           B．does did do 
C．does does do             D．did do doe

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