定语从句由“介词+关系代词”引导
介词后跟有关系代词时要注意：此时的关系代词虽然用作介词的宾语，但不能省略，可以是which，但不能是that。另外，某些表示时间、地点或原因的“介词+关系代词”结构可以与相应的关系副词when, where, why 互换。例如：
This is the pan in which I boiled the milk. 这是我煮牛奶的锅。
I wanted to find someone with whom I could discuss music. 我想找一个能和我谈音乐的人。
The documents for which they were searching have been recovered. 他们找寻的文件已经找到了。
These are the reasons for which [why] we do it. 这些就是我们这样做的理由。
The period in which [when] man learnt to make tools of iron is called the Iron Age. 人类学会制造铁器的时期称为铁器时代。
