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，在从句中作主语和宾语。例如：

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)