一、语法作用：“介词＋关系代词”在从句中主要起关系副词的作用，即在定语从句中充当状语：
1.表示地点，时间和原因的“介词＋which”分别相当于where，when，why。
例句：Is there any reason for which／why you should have a holiday？
2.way后常用that代替in which，也可省略that。
例句：Do it the way I showed you.
二、“of＋which”起形容词的作用，相当于whose（用来指物），其词序通常是“n.＋of which”。
例句：He”s written a book whose name／the name of which I”ve com- pletely forgotten.
三、可引导限制性定语从句和非限制性定语从句
例句：There is a rocket by which the direction of the satellite can be changed.
四、介词的选择（选择介词时须考虑下列情况）
1.介词与先行词的习惯搭配
例句：There is no way in which it could be brought back to the earth.（in与way是习惯搭配）
2.介词与定语从句中的动词，形容词的习惯搭配
例句：These are the wires with which different machines are connected.（be connected with是习惯搭配）
3.同形的先行词，或定语从句中的动词，形容词表示不同的含义时，要根据其含义选用不同的介词。
例句：That is the age in which people live in peace and happiness.
age在前句中意为“年岁”，与at搭配；在后句中意为“时期”，与in搭配。
4.复杂介词须保持其完整形式，常见的复杂介词有on the top of，in the middle of，in front of，in spite of，at the back of，because of等。
例句：We took a photo of rocket，the length of which was about 30me- tres.（SEFC 2A L. 55）
5.短语动词中的介词不可与动词分开提至关系代词前。常见的短语动词有listen to，look at，depend on，pay attention to，take care of，look into，break into，get rid of，take part in，make use of，take hold of，catch hold of，catch up with，get along with，look forward to等。此时可用which，who，whom或that，也可将他们省略。
例句：This is the girl whom he will take careof.
6.当介词与定语从句中的动词或形容词联系紧密（但不是短语动词）时，介词也可放在动词或形容词的后面。但当介词与从句中的动词或形容词联系不紧密时，则需将介词置于关系代词前。比较：
例句：I”ll never forget the day in which I joined the Party.
后句中的in若置于句尾则有“孤零零”之感，这时宜将介词置于关系代词前；而前句中的to则可后置。
五、关系代词的选择
用于介词后的关系代词只有which，whom，前者指物，后者指人。
例句：There are many research stations on the earth in which outer space is studied.（which指代research stations）
