关系代词whom或which，在定语从句中作介词宾语时，介词可放在后面，也可提前构成介词+关系代词whom (先行词指人)/which(先行词指物)引导定语从句。
介词提前常见的几种情况：
where=in/on which
例:
This is the room where / in which he lives.这是他住的房间。
I can see the playground where / on which they do sports.我能看见他们进行运动的操场。
when= on/in/at which
例:
I still remember the year when/ in which I joined the Party.我仍然记得入党的那一年。
I still remember the day when/on which I joined the Party. 我仍然记得入党的那一天。
I still remember the hour when/at which the rocket was launched.我仍然记得那个火箭发射的时刻。
why=for which
例:
This is the reason why / for which he was killed.这是他被杀害的原因。