in which:在......里面；on which:在......的上面，或具體時間的某一天

on which 和 in which 都可以用來引導定語從句，修飾先行詞，區(qū)別在于介詞on/in 這取決于先行詞的搭配

比如:

I can't forget the day on which we had a party.這個句子，先行詞是day，原本的搭配是on the day.

I like the room in which there are many toys.這句話的先行詞是room，詞組搭配是in the room。