in which:在......里面；on which:在......的上面，或具体时间的某一天
on which 和 in which 都可以用来引导定语从句，修饰先行词，区别在于介词on/in 这取决于先行词的搭配
比如:
I can't forget the day on which we had a party.这个句子，先行词是day，原本的搭配是on the day.
I like the room in which there are many toys.这句话的先行词是room，词组搭配是in the room。
例句：
1.The school (that/which) he once studied in is very famous.
= The school in which he once studied is very famous.
2. 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.
固定搭配：
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 little creature in which scientists are interested is known as ET.
(这里"be interested in"是词组原因)
例题：
I can’t believe that the restaurant, ______ I have eaten such wonderful meal, is going to be close down. A. where B. at which C. about which
【解答】选B。这个是非限制性定语从句，at which在功能上等于where，但就本题而言只能选at which。因为at which I have eaten such wonderful meal是一个插入语，where固然可以引导，但是由于前面有了一个逗号，就只能用at which了。“我难以相信我曾经在里面美美地吃过几顿大餐的饭店就要关张了。”非限制性定语从句主要就是先行词+逗号“，”+介词+which或者是先行词+逗号“，”+which，它和限定性定语从句的最显著区别一个是有逗号，一个是非限制定语从句总有which。