一、关系副词引导的定语从句
关系副词可代替的先行词是时间、地点或原因的名词，在从句中作状语。定语从句中的关系副词有三个：when, where和why，它们在定语从句中分别作时间、地点和原因状语。
上述三个关系副词很多情况下都可以用介词＋关系代词来替换。where＝in/at/to＋which; when＝in/on/at＋which; why＝for＋which。
[合作探究] 阅读下列句子，体会定语从句的用法
①But it was also a time when there were many great philosophers.
②Ancient China was a place where states were often at war with each other.
③Do you know the reason why he has been late?
