定语从句中常用的关系副词有：when，where，why，在定语从句中分别作时间状语，地点状语和原因状语。
1.when的用法
when代替的先行词是表示时间的名词，在定语从句中作时间状语。when往往可以由介词+which代替。
Can you tell me the time when the film will start? 你能告诉我电影什么时候开始吗?
Can you tell me the time at which the film will start?
The solution to the mystery of the monarch’s amazing ability comes at a time when it is in serious trouble. 金斑蝶惊人能力之谜的谜底在它陷入严重困境之际揭晓。（新外研版必修一）
（when可以用during which代替）
2.where的用法
where代替的先行词是表地点的名词，在定语从句中作地点状语。where往往可以由介词+which代替。
Eventually, it manages to reach the places where it will spend the winter. 最终，它设法到达了过冬的地方。（新外研版必修一）（where可以由in which代替）
They were welcomed to the Chinese-style compound where they were to live. 他们受到了欢迎，住在中国式的大院里。（新外研版必修一）（where可以由in which代替）
where的先行词也可以是一些类似于地点的抽象名词，例如，case（情况，案例），situation（情况，形势，境遇），degree（程度），point（时刻，阶段），job（工作），career（事业），family（家庭）等。
We have reached a point where a change is really needed. 我们已经到了一个真正需要改变的时刻。
I don’t want a job where I’m always ordered to do the work all day. 我不想要一份总是被命令整天干活的工作。
We have got into a situation where we were supposed to make a decision immediately. 我们陷入了一种应该立即做出决定的境地。
All the neighbors admire this family where the parents are treating their child like a friend. 所有的邻居都很羡慕这个家庭，他们的父母像对待朋友一样对待他们的孩子。
3.why的用法
why代替的先行词通常是the reason，在定语从句中作原因状语。
Sadly, human activity is the main reason why the number of monarch butterflies is falling. 可悲的是，人类活动是黑脉金斑蝶数量下降的主要原因。（新外研版必修一）
And of course, the main reason why they were so excited is that pandas are just so cute. 当然，他们如此兴奋的主要原因是熊猫太可爱了。（新外研版必修一）
4.关系代词和关系副词的选择
在定语从句中是选择关系代词还是关系副词，取决于定语从句中缺少什么成分，如果缺少主语，宾语，表语，定语，需选择关系代词（who，whom，that，which，whose，as等）；如果缺少状语，需选择关系副词（when，where，why）
I will never forget the days when we stayed in the countryside. 我永远不会忘记我们在农村度过的日子。（定语从句中缺少时间状语）
I will never forget the days that /which we spenttogether. 我永远不会忘记我们一起度过的日子。（定语从句中缺少spent的宾语）
That was the reason why he was late. 那就是他迟到的原因。（定语从句中缺少原因状语）
The reason that you gave us was not reasonable. 你给我们的理由不合理。（定语从句中缺少宾语）
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