（1）when
when在定语从句中作时间状语。可由at/on/in/during which替代。
April Fool's Day is the day when (= on which ) people make fun ofothers.
愚人节是人们愚弄他人的日子。
Do you know the date when (= on which ) Lincoln was born?
你知道林肯的出生日期吗?
（2）where
where在定语从句中作地点状语，可由at/on/in/from/to which替代。
These are the few points where（= at which）we disagree with each other.
这些就是我们之间有分歧的几点。
This is the mountain village where (=in which) I stayed Iast year.
这就是我去年待过的山村。
He did all his research in the room where (=in which) he lived.
他在自己住的房间里做了所有的研究。
（3）why
why在定语从句中作原因状语，可以用 for which代替。why不可引导非限制性定语从句，此时可用for which。
I have come around to explain the reason why（=for which）I was absent from the meeting.
我来是为了解释我当时没有到会的原因。
Is this the reason why（=for which） she refused our offer?
这就是她拒绝我们帮助她的理由吗?
知识延伸
the reason why...与 the reason that
1)下面的句子是that引导的同位语从向，that不作成分，that后面的内容是解释说明 reason的内容的，而why引导的定语从向并不解释说明 reason的内容。
I don’t want to hear any reason why you were absent.
我不想听你缺席的任何理由。（why引导定语从句，在从句中作原因状语）
The reason that the car broke down dosen’t hold water.
车子坏了这一原因是站不住脚的。（that引导同位语从句，解释reason的内容，在同位语从句中不作成分）
2）在下面这个句子中，不能用why，只能用which或that。
I don’t want to hear any reason (that/which) you might give.
我不想听你可能给出的任何理由。(在这个定语从句中，that/ which作定语从句中谓语动词give的宾语)
注意：以下的关系副词可省略
1 ) the time, every time, each time, the moment后的关系副词可省略。
By the time (when) he was fourteen years old, Einstein had learnt advanced mathematics all by himself.
到十四岁时，爱因斯坦已经自学了高等数学。
2) 在某些表示地点的名词后，关系副词有时也可以省略，如 the place等。
This is the place ( where) we met years ago.
这就是我们多年前相遇的地方。
3) 先行词the reason后面的关系副词可省略。
The reason he did that is quite clear.
他那样做的理由是非常清楚的。
- 主谓一致问题
关系代词在定语从句中作主语时，定语从句中谓语动词的单复数形式与先行词应一致。
（1）先行词是单数，从句的谓语动词用单数; 先行词是复数，从句的谓语动词用复数。
Mr Smith, who is now downstairs, is asking to see you
史密斯先生要见你，他现在在楼下。
The Smiths, who are now downstairs, are asking to see you.
史密斯夫妇要见你，他们在楼下。
（2） “one of 复数名词”后的定语从句的谓语动词用复数形式;＂the (only) one of 复数名词” 或“the very/ right one of 复数名词”后的定语从句的谓语动词用单数形式。
Jeff is one of the students who were awarded.
杰夫是那些获奖的学生中的一个。
Jeff is the only one of the students who was awarded.
杰夫是那些学生中唯一一个获奖的。
- what不可引导定语从句
what本身是“复合关系代词”，等同于“先行词 关系代词( which/that)”。因此不能用what引导定语从句。
Tell me what is happening. =Tell me all that is happening.
告诉我正发生着什么事。
She isn’t what she used to be. =She isn’t the one (that) she used to be.
她不是过去的她了。
She did all what she could to help the poor (x)
She did all ( that ) she could to help the poor. (V)
She did what she could to help the poor. (V)
地尽其所能帮助穷人。
