前面介词一定是看定语从句中动词来确定吗?比如from which she come 还有介词+which whom一定可

介词加关系代词 引导定语从句 与介词的用法有关 当然要看与动词的搭配.不过一些固定点动介词组 一般是不分开的 也就是介词不提前.如 this is the pen which he has been looking for.这就是他一直在找的那支笔 一般不说 for which he has been looking for.介词加关系代词 在定从中作状语 要比 关系副词where；when在定从中作状语 具体 也就是说 关系副词范围要大于 介词加关系代词.所以有时不能互换.如this is the desk where i found the book.这是我从那儿找到这本书的桌子.这就比较模糊 是桌子上?还是桌子里?所以this is the desk on/in which i found the book .这是我在上面/在里面 找到这本书的桌子.我认为这句的in/on which就不能和where互换.