（一）定语从句一般由关系代词和关系副词引导
1、关系代词：who, whom, whose, which, that, as
2、关系副词：when, where, why
关系代词和关系副词必须位于从句之首，主句先行词之后，起着连接先行词和从句的作用， 同时在从句中又充当句子成分。
e.g. She is the girl who sings best of all.（关系代词who在从句中作主语）
The comrade with whom I came knows French.（whom在从句中作介词with的宾语）
