介词前置定语从句，介词可以不提前吗

比如The time of year in which you were born decides your star sigh.
先行词为the time of year, 生于...词组为be born in...,因此在which 前需要加介词in. 又如this is the room which I once lived in/this is the room in which I once lived. 介词提前或不提前都可以.

楼上的搞错了吧...

一般来说如果介词属于一个有特别意义的固定词组的话就不提前，像
this is the book which he is looking for. looking for 固定搭配为寻找的意思 look 跟for的搭配有了一个特定的意思 所以 不提前for

而像楼上举得
this is the room which I once lived in/this is the room in which I once lived. live in 没有什么特别的意思是住在.... 这里的live没有跟in一起 形成一个词组 有新的意思 所以一般要提前

当然可以！
举个例子吧
This is the house in which i lived 3 years ago.
This is the house which i lived in 3 years ago.
This is the house where i lived 3 years ago.