里有很多 who whom whose that which
它们都是用来引导定于从句的.这类代词都起着三重作用,一是代表前面的名词,而是把从句和它所修饰的词连接起来.三是在从句中也担任一个成分,例如：
*I met someone who said he knew you.我碰到一个人,他说他认识你.
（who代someone,又引导从句修饰它,同时又在从句中担任主语.整个从句是“who said he knew you”）
*the noise that he made woke everybody up.他弄出的声音把大家都吵醒了.
（that代noise,又引导从句修饰它,又在从句中担任made的宾语.）
***这种代词起着关联作用,把引导的从句和所修饰的词连接起来,因此称为关系代词,他说引导的从句称为关系从句,也称为定于从句.
**who,whom和whose的用法
1）who代表人,在从句中做主语.
The man who robbed her has been arrested.抢她东西的人被捕了.
The girl who served in the shop were the owner' s daughter.在商店里接待顾客的那些姑娘是老板的女儿.
2）whom也代人,但在从句中做宾语或介词宾语.
The man whom i saw told me to come back today.我见到的那个人叫我今天再来.
The girls whom he employs are always complaining about their pay.他雇用的姑娘们老是抱怨工资太低.
The man with whom i was travelling didn't speak English.和我一道旅游的那人不会讲英语.
前两句的whom在从句中做宾语；后一句做介词宾语,因为它跟在with后面（你可以这么简单的理解）.
*****怎样来判断一句句子里什么时候用WHO,什么时候用WHOM?
这时候你需要考虑把句子还原回去.由于who和whom都是代人的,所以你把他们一起看成somebody.比如第一句,The man who robbed her has been arrested.你把中间那个who不看,当它不存在,然后用somebody来代替,因为那个somebody是用来代The man的,所以The man不看也行.这是这句话就是 somebody robbed her has been arrested.这句话的语序还是对的,所以是成立的,因此这时用who.
The girls whom he employs are always complaining about their pay.这句中,同样把The girls不看,把who看成somebody,那句子就是somebody he employs are always complaining about their pay.这样的话语序就错掉了,正确的是 he employs somebodys are always complaining about their pay.这时你会看到somebody在一个谓语的后面当宾语,所以这时用whom.
The man with whom i was travelling didn't speak English.这句话你把它变过来就是i was travelling with somebody.这时跟在介词后面,也用whom.而这句话中的didn't speak English也是修饰the man的,The man who didn't speak english.这整个一句话就是把“the man with whom i was traveling.”和“the man who didn't speak English.”合起来的.要是你不理解的话,我下次再和你讲一遍
3）whose表示“某人的”,在从句中做定语.
The film is about a spy whose wife betrays him.这部电影讲一个间谍,他的妻子出卖了他.
A mental patient is one whose mind is diseased.精神病人就是脑子有病的人.
I know a boy whose father ia an acrobat.我认识一个男孩,他父亲是杂技演员.