在什么情况下定语从句的关系代词前要加介词?经常发现很多选项的关系代词前有介词,因为我还没有系统学过这些内容,就不太会做了,还有,定语从句中关系代词的选用有哪些注意事项和容易混淆的东西等,有哪些特殊情况需要注意,有例子就更好了.说得通俗一点.

有时候根据动词的习惯搭配和先行词的习惯搭配来选择介词。

定语从句关系代词前的介词，并不是由我们选用的，而是取决于句子中谓语动词之间的搭配关系。通俗的讲，这些介词是谓语动词本身带来的，只不过有时为了句子的平衡而把它们放到了关系代词之前而已。如：
This is the house in which I lived two years ago.原本是This is the house which I lived in two years ago。live in 就是从句的谓语动词本身是搭配关系。
再如：The house,the windows of which face the south, is very expensive.

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 .