1.我想有介词+ which 是因为很多情况下where与when用在句中不能代替介词+which。例如：This is a good pen which is comfortable to write with.
可变为介词提前的：
This is a good pen with which is comfortable to use.
很显然，with which 不能用 where 或when 或why 或whose 代替。2.有以上例句可知，当定从中动词为不及物动词时后必须加介词。介词可以提前到which前时用介词+which。...
