这些都是定语从句里面，由which引导的定语从句，介词提前。
in which 可以翻译成在......里面
for which 可以翻译成为了......目的
on whch 可以翻译成在......的上面
at which 可以翻译成在.....里面或在.....上面
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
例如 (1) The school (that/which) he once studied in is very famous."
= The school in which he once studied is very famous.
(2) Tomorrow I will bring here a magazine (that/which) you asked for.
= Tomorrow I will bring here a magazine for which you asked.
(3) 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.
（4 ) The dumpling House which he often has dumplings at is very popular.
= The dumpling House is very popular, at which he often has dumplings.
当然这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。
例如 The plane may be several hours late,in which case there's no point in our waiting.
（固定搭配：in the case)
The speaker paused to examine his notes,at which point a loud crash was heard.
( 固定搭配：at the point)
Ten 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)
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