三、关系副词的用法
 

　　（1）when指时间，其先行词表示时间，在句中作时间状语。例如：
This was the time when he arrived. 这是他到达的时间。

　　（2）where指地点，其先行词表示地点，在句中作地点状语。例如：
This is place where he works. 这是他工作的地点。

　　（3）why 指原因，其先行词是原因，起原因状语作用。例如：
Nobody knows the reason why he is often late for school. 没人知道他为什么上学总迟到。

　　四、which 和 as

　　在定语从句的非限制性定语中，which 和as 是容易混淆的。在很多情况下两者兼可互换，但在下列情况中不可互换：

　　（1）通常as 可以放在整句的句首，而which 只能在逗号之后，as 本身含有“正如”，as 在定语从句中既可是主语也可做宾语。
As appear from her paper, she has read widely in Romantic literature.
She is remarkable, as I have told you .

　　（2）which 在做非限制性定语从句关联词时可以无明确先行词，指前句整个的意思，如：She has warried again, which surprises us.

　　五、all that 和 what

　　在定语从句中，许多人往往分不清all that 和what，what 实际上只引导名词性从句，它相当于all that 两个字，例如：
All that I know is that he has made up his mind to heave the country for a new place.
= What I know is that he has...