1、关系代词that ,which ,who 和whom在定语从句作主语，宾语或表语，也就是说，如果在定语从句中，缺少这些成分的话，就用这些词。关系代词whose在定语从句中做定语。关系副词where ,whenh和why在定语从句中充当状语。我们知道，定语从句的结构是（先行词）+关系词+定语从句。关系词的选择主要看定语从句中缺少了什么成分。
2、如:This is the factory ______his father worked ten years ago.在his father worked ten years ago部分，work是一个不及物动词，明显缺少了地点状语，只能用关系副词where。
3、再如：The boy _____ name is Tom didn't come to school yesterday.在_____ name is Tom中，缺少了name的定语，所以用关系代词whose。又如：This is the man ______took my pen yesterday、在______took my pen yesterday中，明显缺少了主语，所以用who或that。