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。
3)which, that
它们所代替的先行词是事物的名词或代词，在从句中可作主语、宾语等，例如：
A prosperity which / that had never been seen before appears in the countryside.农村出现了前所未有的繁荣。(which / that在句中作宾语)
The package (which / that) you are carrying is about to come unwrapped.你拿的包快散了。(which / that在句中作宾语)
