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)
