定语从句在句中做定语,修饰一个名词或代词,被修饰的名词,词组或代词即先行词.定语从句通常出现在先行词之后,由关系词（关系代词或关系副词）引出.
关系代词有：who, whom, whose, that, which等.
关系副词有：when, where, why等.
关系代词所代替的先行词是人或物的名词或代词,并在句中充当主语、宾语、定语等成分.关系代词在定语从句中作主语时,从句谓语动词的人称和数要和先行词保持一致.
1）who, whom, that
这些词代替的先行词是人的名词或代词,在从句中所起作用如下：
Is he the man who/that wants to see you?
他就是你想见的人吗?（who/that在从句中作主语）
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）
2) Whose 用来指人或物,(只用作定语, 若指物,它还可以同of which互换）, 例如：
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.
Please pass me the book whose (of which) cover is green. 请递给我那本绿皮的书.