定语从句语法问题英美概况上的一句话.Lurking beneath the surface of this portry

介词提前应该不仅仅限于从句中有不及物动词,只要是关系代词需要和介词搭配,或者谓语动词需要和介词搭配的情况
在定语从句中,一般有两种情况会在关系代词前出现介词：
1、与先行词有关的介词可以放到关系代词前,如：
I won't forget the days in which we worked together.(因为in the days,所以 in which)
2、与定语从句中的谓语动词相关的介词会放到关系代词前,如：
The boy about whom they often talk is very kind.(因为talk about,所以about whom )
注：第二种情况是完全可以将介词放到原有的位置（定语从句的谓语动词后）,这时关系的范围就大了很多,如The boy whom/who/that/不填 they often talk about is very kind.
如果介词在关系代词前,关系代词要么是which（先行词是物）,要么是whom（先行词是人）.
希望能对你有所帮助.
以上复制自sdm_a1的回答
补充,可以理解成状语前置