定语从句中的who,whom,which,that,what,it,as,怎么用,总是分不清什么时候用哪个

一、关系代词who
who表示“……的(人)”,在定语从句中作主语,不能省略,所引导的定语从句一般修饰指人的名词或代词,在口语或非正式语言中,可作定语从句中的宾语,相当于whom,可省略.
These are the workers(who/whom) he employs. 这些是他雇佣的工人.
This is Uncle Li who mended computers for us. 这就是为我们修电脑的李叔叔.

二、关系代词whom
whom表示“……的(人)”,在定语从句中作宾语,所引导的定语从句修饰指人的名词或代词,常省略.
Mr. Wang is the man(whom) we all respect. 王先生是我们大家都尊重的人?
The soldier (whom) you want to see has already come. 你要见的那名士兵已经来了.

三、关系代词that
that表示“……的(事物)”,所引导的定语从句修饰指事物或人的名词或代词,通常在定语从句中作主语或宾语,作宾语时,常可省略.
This is the best film that I have ever seen. 这是我看过的电影中最好的一部.
The book (that) I am reading is very interesting. 我正在看的这本书非常有趣.

四、关系代词which
which表示“……的(事物)”,所引导的定语从句修饰指事物的名词或代词,其中which通常在定语从句中作主语或宾语,作宾语时,常可省略.
He works for a company which makes washing machines. 他在一家生产洗衣机的公司工作.
Take away the pencil which is lying on the desk. 把课桌上的那支铅笔拿走.
The money (which) they found in the street is mine. 他们在街上捡到的钱是我的.

五、关系代词whose
whose表示“那个(那些)人的、它（它们)的”时,所引导的定语从句修饰指人或事物的名词,其中whose在从句中作定语,修饰名词 如：
There is nobody whose name is Wang Pengtao here. 这儿没有名叫王鹏涛的人.
The aunt whose photo I showed you yesterday will come to see us this evening. 昨天我给你看她的照片的阿姨今晚要来看我们.
六、as既可指人,也可指物,常在限制性定语从句中充当主、宾、表语.as引导的定语从句的先行词前面常有as ／such ／the same等修饰词.例如： ①He is as brave a man as ever lived．他是世界上最勇敢的人. ②Don’t read such books as are not worth reading．不要读那些不值得读的书. as作关系代词,还可用来引导非限制性定语从句,as引导的定语从句可放在句首、句中或句末.例如： ① Taiwan,as we all know,is an inseparable part of China． ②As we all know,Taiwan is an inseparable part of China． ③Taiwan is an inseparable part of China, as we all know．
声明：
what,it不是关系词,不能用在定语从句中.