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引导定语从句的关系副词有三个：when,where,why,都在定语从句中担任状语。when 既可引导限制性定语从句，又可引导非限制性定语从句。where既可引导限制性定语从句，又可引导非限制性定语从句。why只能引导限制性定语从句。
定语从句关系副词用法
关系副词when的用法
when 既可引导限制性定语从句，又可引导非限制性定语从句。先行词是时间名词，在从句中充当时间状语，相当于“介词+which”。如：
Can you tell me the time when the film will start?—Can you tell me the time at which the film will start? 请告诉我电影什么时候开始好吗?(限制性定语从句)
I will never forget the day, when I joined the Party.—I will never forget the day, on which I joined the Party. 我入党的那一天，我永远不会忘记。(非限制性定语从句)
关系副词where的用法
where既可引导限制性定语从句，又可引导非限制性定语从句。先行词是地点名词，在从句中充当地点状语，相当于：“介词+which”。如：
This is the school where I studied a few years ago.—This is the school in which I studied a few years ago. 这就是我几年前所在读书的学校。(from www.yygrammar.com)
Let’s go to the concert, where you will find much fun.—Let’s go to the concert, in which you will find much fun. 我们去音乐演唱会吧，在那里你会找到很大的乐趣。
关系副词why的用法
why只能引导限制性定语从句，先行词只有reason一词，在从句中充当原因状语，相当于：“for + which”。如：
Do you know the reason why he is not here now?—Do you know the reason for which he is not here now?你知道他还没有来这儿的原因吗?