在什么情况下定语从句的关系代词前要加介词?

第一,为了使句意完整如
I have many friends,some of whom are businessmen.
Mr Black has two sons ,both of whom like sports.
第二,定语从句中谓语是由动词加介词构成的短语,可把介词提到关系代词之前如
The man with whom my father is talking is our teacher.
定语从句最主要的是要分清关系代词和关系副词的区别.代词有which that who whom whose as它们在从句中作主语,宾语,表语或定语,所以当从句中缺少其中某一种以上成份时就选择关系代词.
This is the city that/which we visited last year.(这句中缺少visit的宾语)
Anyone who breaks the law must be punished.(这句中缺少主语且表人)
The book that/which I bought yesterday is very interesting.(这句中缺少主语且指物)
The boy whose father is a lawyer is our monitor.(这句中缺少定语修饰father)
I live in the house whose door faces the south.(同上)
Kate is no longer the girl that she used to be.(作表语)
如果先行词在句中作状语用关系副词引导,有where when why
This is the city where Tom was born.=Tom was born in the city所以说先行词在从句中作关语.
I will never forget the days when we worked together.先行词在从句作时间状语.
The reason why he was late is that he was ill .