定语从句中什么时候用in which什么时候用which.

给你两个例子吧.
in which：It's the restaurant in which I met Mary yesterday.
which:I like the house which color is beautiful.
简而言之,which单独用的时候只能作为定语从句中的主语/宾语来修饰先行词,也就是关系代词.
然而和介词搭配的时候,就变成了介词短语,可以当作先行副词来使用,例如上句的in which就等於where,然而这里的which代表的是restaurant,是介词in后面的宾语,而where表示的是in the restaurant~
for which 和with which的用法是一样的,也举两个例子吧~
for which：It's a psp for which it takes me 2000 yuan.
with which：He bought a pen with which he can write.
再拿for which这道题来解释吧,还原的话就是It takes me 2000 yuan for the psp.看到了麽,介词+关系代词中的介词用法主要是看句型和短语的,至於后面到底是用which/who就要看修饰的是人或物了.
最后一例：
It's a psp on which I spent 2000yuan.
因为是spend money on sth.,所以这里就是on which~
不知道这麽说你能不能懂……实在不行的话,请去买一本语法实践指南…= =|||