（3）Often, the reason why people are unhappy is that they do not have enough love.

(关系副词作原因状语)通常人们不快乐的原因是因为他们得不到足够的爱。

△非限定性定语从句：非限定性定语从句是对先行词的附加说明，若省去也不影响主句的意思，它和主句间常用逗号隔开。

e.g. (1) The Yangtze River is the third longest river in the world, which is more that 6, 000 kilometres long. （关系代词作主语）长江是世界上最长的河，它有6000多米长。

（2）Sun Yat-sen, who was the leader of 1911 Revolution, first suggested the idea in 1919. (关系代词作主语)孙中山是最先在1919年提出这个思想的人，他是1911年辛亥革命的领导者。

注意：

（1）介词放在关系代词的前面时，介词宾语只能用which代物，用whom代人。

e.g. He is a man of whom China can be proud. 他是中国引以为自豪的人。

This is the house in which the inventor lived.这就是这位发明家曾经住过的房子。

（2）在限定性的定语从句中，当关系代词在从句中担任动词宾语时，关系代词可省略。

　　e.g. The dam (that) we saw in the film wasn't the Three Gorges dam.

　　我们在电影中看到的大坝不是三峡大坝。

The students (that) I met near the reservoir were from Vietnam.

我在水库附近遇到的学生们来自越南。

（3）在限制性定语从句，当关系代词在从句中担任介词而介词在句尾时，关系代词可以省略。

e.g. I met a man (who) my grandfather worked with thirty years ago.

　　我遇到了30年前和我爷爷一起工作的那个人。

　　I wanted to visit the house (that) my grandparents lived in.

　　我想去看看我祖父母曾居住过的房子。

（4）有时为了行文需要，定语从句中的关系代词和部分谓语动词可以省略。

e.g. The people (who were) living in the village have moved to other places.

　　　以前住在村子里的人们已经搬到其它地方去了。

　　　They came from a village (that was) submerged in the reservoir.

　　　他们来自一个被淹没在水库里的村子。