who,whom,that
这些词代替的先行词是人的名词或代词,who做主语指人,whom作宾语指人,that既可作主语又可作宾语（作宾语可以省略）,可以指人也可以指物.在从句中所起作用如下：
(1) Is he the man who/that wants to see you?他就是想见你的人吗?（who/that在从句中作主语）
(2) He is the man whom/ that I saw yesterday.他就是我昨天见的那个人.（whom/that在从句中作宾语）
Whose 用来指人或物
(只用作定语,若指物,它还可以同of which互换,指人的时候也可以用of whom 代替）
(1) They rushed over to help the man whose car had broken down.那人车坏了,大家都跑过去帮忙.
(2) Please pass me the book whose (of which) color is green.请递给我那本绿皮的书.