1.由who引导的定语从句中，who用作主语，如：This is the boy who often helps me. 2.由whom引导的定语从句中，whom用作宾语，如：The man whom you are waiting for has gone home. 实用文档
3.由whose引导的定语从句中，whose用作定语，如：Do you know the girl whose skirt is white? 4.由which引导的定语从句中，which用作主语或谓语动词的宾语或介词的宾语，如：
The room in which there is a machine is a work shop. The river which is in front of my house is very clean. This is the pen which you want. 注意：
(1whom, which用作介词宾语时，介词可放在whom、which之前，也可放在从句原来的位置上；但在含有介词的动词固定词组中，介词只能放在原来的位置上。如：He is the very person whom we must take good care of. (2引导非限制性定语从句时，必须用关系代词which，不用that,如：I have lost my bag, which I like very much. (3关系代词在句中作主语时，从句的谓语动词的人称和数必须和先行词保持一致。
