其实不是介词不可以提前到关系代词前,二是有些关系代词前面不能有介词.That就是不可以的,但是which和who whom都可以在介词后面.比如说,I know the man with whom your father is talking 是对的,可使用that的时候,就只能说I know the man that your father is talking with了.
如果不明白的话,欢迎一同再次交流.