which 用于定语从句，作关系代词，在句中要做成分
in which 用于定语从句，作关系副词，在句中不做成分，in which=where

in which 只能用在定语从句中，等于where,在定语从句中作状语。
如which的用法：He lived in the house in which Tom once lived.
He lived in the house where Tom once lived.

定语从句中.如
this is the room in which we stayed
先行词是room，后的句子是用来修饰room的
但是room不可做stay的成分，因为stay是不及物动词，后不可直接家宾语
所以要有个介词．
其实上述的句子＝
this is the room which we stayed in．
这里的介词是可以提到which 前的

注意，第二个句子可用that 或省略

但第一个只能用which．只有which前可加介词