This is the house. He has lived in the house for over 30 years.
变为↓定语从句
This is the house in which he has lived for over 30 years.
上述句子中，先行词在从句中充当介词宾语，形成"介词 which / whom"的形式，考试时要求作出1)要不要介词；2)若要，使用什么介词的判断。由于英语中介词搭配多为习惯用法，没有多少规律可循，因此这是定语从句考试的一个难点。
记住四个字：瞻前顾后
瞻前：先看看先行词一般与什么介词搭配，例如：
There are occasions _____ which joking is not permissible.
Joking is not permissible _____ (certain) occasions.
常与occasion搭配的介词是什么?on， 即：
Joking is not permissible on (certain) occasions. (有的场合是不容许开玩笑的。)
↓
There are occasions on which joking is not permissible.
顾后：看看谓语动词常与什么介词搭配，例如：
This car， ____ which I paid a lot of money， is now out of date.
I paid a lot of money ______ this car.
Pay (money) for something 是一个固定的搭配，因此应填介词 "for"
当A、B、C、D中出现 " 介词 which / whom" 选项时，而你又确实看不懂时，应倾向于选择" 介词 which / whom"的形式，例如：
The two elements _____ water is made are the gases oxygen and hydrogen.
A. that
B. which
C. with which
D. of which
倾向选择C、D;正确答案D，"be made of "(由…组成):
The two elements are gases (oxygen and hydrogen).
Water is made of the two elements.
两个句子一合并，便是该句选择。
(四)其他应注意的确问题：
(1) 同位语从句只能用"that"连接
常见的同位语先行词有：fact(事实)， possibility(可能)， idea(想法)， belief(相信)， doubt(疑问)， news(消息)， order(命令)， promise(承诺)， evidence(证据)， suggestion(建议)等，例如：
Is there any possibility that all the villagers can send their children to school?(有没有这种可能，所有村民都能送他们的孩子上学?)
There can be no doubt that he is a qualified doctor.(毫无疑问，他是个合格的医生。)
(2) "as" 也可以做关系代词用于连接定语从句，特别是在"such…as…"， "the same as…" 以及"正如…"的结构中，例如：
I have never seen a thing as he described. (我从未见过他所描述的那种事。)
As was expected， the performance was a great success. (正如所料，演出获得了巨大成功。)
把该句理解为：The performance was a great success， which was expected，就比较容易理解as的这种用法。
将定语从句的连接用三句话归纳一下：
↗ 人用who / whom
(1)先行词在从句中当主、宾、表语 定语用whose
↘ 物用that (= which)/ 非限制用which
↗地点状语用：where
(2)先行词在从句中当 原因状语(reason)用：why
↘ 时间状语用：when