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3. 一点注意: 注意不要以为在时间名词后就一定用关系副词when， 在地点名词后就一定用关系副词 where，在表示原因的the reason/cause后就一定用关系副词why。到底选用关系副词还 是关系代词，关键是看它们在定语从句是用作状语(用关系副 词)还是用作主语或宾语(用关系代词)。
如：Don’t forget the time (that/which) I’ve told you. 不要忘记我告诉你的时间。(that用作told的宾语) I’ll never forget the days when I worked with you. 我 永远不会忘记我与你共事的日子。(when用作状语)
2.先行词问题 关系副词when和where的先行词是表示时间(如the day, the days, the time, the year等)和地点(如the place, the house, the factory, the station等)的一类词，而关系副 词why的先行词通常只是 reason(s)/cause(s)。另外，when和 where可引导限制性和非限制性定语从句，而why则只用于引导限 制性定语从句，不用于引导非限制性定语从句。
I will never forget the day when I first met her. = I will never forget the day on which I first met her.
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He works in a factory that /which makes radio parts. 他 在一家制造无线电零件的工厂工作。(that, which 在从句中 用作主语，且不能省略)
He works in the factory where his father worked. 他在他 父亲工作过的那家工厂工作。(where作状语)
由关系副词when, where, why引导的定语从句
1. 基本用法:定语从句除可用关系代词which, what,who,whom,whose引导外，还可用关系副词when, where与
why来引导，它们的区别是：when用于指时间，where用于指地点， why用于指原因。when, where, why 在定语从句中均用作状语。
That’s the reason that /which he explained to us. 那就 是他向我们解释的理由。(that或which在从句中用作 explained的宾语)
That’s the reason why she left home. 那就是她离家出走 的原因。(why在从句中作状语)
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关系副词when,where,why == 介词+关系代词,当把先行 词放回从句中，要加上一个介词，就用一个关系副词， 不需要加时，就用which.
一般 where = in/at/ with +which. when = on which why = for which. The hotel where we stayed wasn’t clean. The hotel at which we stayed wasn’t clean. The hotel (which) we stayed at wasn’t clean.