老师说 定语从句中地点状语in which =where 我想知道是怎么转化过来的 首先说说我的猜想

老师说 定语从句中地点状语in which =where 我想知道是怎么转化过来的 首先说说我的猜想
老师说 定语从句中地点状语in which =where
猜想：I‘ll never forget the day on which I joined the club 中on which是不是这么得出来的呢：∵on the day ∴on which也就是说which前的介词以先行词前所需加的介词决定吗?这结论对吗 那么 这 里 的 on which 等于when
句子1 Don't forget the day when you were received into the Youth League中的when 是不是也能用on which代替呀?
句子2；Do you know the reason why she has changed her mind?这里why又等于什么了呢?A why B which C for that D of which
C项 的for that 等于reason吗（因为 the reason for 可为什么要用that 了呢?难道where =in which 的which 也可以换成that么?）