in which, for which, on which, at which的不同用法 

　　1)这些都是定语从句里面，由which引导的定语从句，介词提前。下面这些介词的使用除了意思上的区别，具体是要以which引导的从句而定的。 

　　①in which可以翻译成在……里面 

　　The school (that/which) he once studied in is very famous. 

　　= The school in which he once studied is very famous. 

　　②for which可以翻译成为了……目的 

　　Tomorrow I will bring here a magazine (that/which) you asked for. 

　　= Tomorrow I will bring here a magazine for which you asked. 

　　③on which可以翻译成在……的上面，或具体时间的某一天 

　　I have a dresser in my bedroom, which there are many cosmetics on. 

　　= I have a dresser in my bedroom on which there are many cosmetics. 

　　④at which可以翻译成在……里面或在…… 

　　The dumpling House which he often has dumplings at is very popular. 

　　= The dumpling House is very popular, at which he often has dumplings. 

　　2)当然第一点只是介词作为基本意思的用法，还有一些固定搭配，得具体情况具体分析。 

　　①The plane may be several hours late, in which case there’s no point in our waiting. 

　　(固定搭配：in the case) 

　　②The speaker paused to examine his notes, at which point a loud crash was heard. 

　　(固定搭配：at the point) 

　　③Ten years of hard work changed her greatly, for which reason he could hardly recognize her at first sight. 

　　(固定搭配：for the reason) 

　　④The film at which I fell asleep was very boring. 

　　(Here "at which" is based on "fell asleep at the film"因为电影无聊而睡觉。)

　　⑤The little creature in which scientists are interested is known as ET. 

　　(这里"be interested in"是词组原因) 

　　⑥This is the book for which he is looking. 

　　(即等于This is the book which he is looking for，这主要是因为look for 这一短语) 

from:

http://www.tesoon.com/ask/htm/23/114790.htm