which 用於定語從句，作關係代詞，在句中要做成分
in which 用於定語從句，作關係副詞，在句中不做成分，in which=where
in which 只能用在定語從句中，等於where,在定語從句中作狀語。
如：He lived in the house in which Tom once lived.
He lived in the house where Tom once lived.
定語從句中.如
this is the room in which we stayed
先行詞是room，後的句子是用來修飾room的
但是room不可做stay的成分，因為stay是不及物動詞，後不可直接家賓語
所以要有個介詞．
其實上述的句子＝
this is the room which we stayed in．
which 用於定語從句，作關係代詞，在句中要做成分
in which 用於定語從句，作關係副詞，在句中不做成分，in which=where
in which 只能用在定語從句中，等於where,在定語從句中作狀語。
如：He lived in the house in which Tom once lived.
He lived in the house where Tom once lived.
定語從句中.如
this is the room in which we stayed
先行詞是room，後的句子是用來修飾room的
但是room不可做stay的成分，因為stay是不及物動詞，後不可直接家賓語
所以要有個介詞．
其實上述的句子＝
this is the room which we stayed in．