1．定语从句
在复合句中，修饰名词、代词或整个句子的从句叫作定语从句。定语从句可以分为限制性定语从句和非限制性定语从句。
The boy who is reading needs the pen which you bought yesterday.那个在阅读的男孩需要你昨天买的钢笔。(定语从句作后置定语)
2．先行词
被定语从句所修饰的词或句子叫先行词，作先行词的可以是：
(1)一个词(通常是名词，也可以是代词)
This is the place which is worth visiting.
这是值得参观的地方。
He laughs best who laughs last.谁笑到最后，谁笑得最好。
(2)一个短语
Many life's problems which were solved by asking family members, friends or colleagues are beyond the capability of the extended family to resolve.许多以前可以通过询问家庭成员、朋友或者同事就能解决的生活问题，是现在的大家庭无力解决的。
(3)一个分句
The Greeks assumed that the structure of language had some connection with the process of thought, which took root in Europe long before people have realized how diverse languages could be.希腊人认为，语言结构和思维过程之间存在着某种联系。这一观点在人们尚未认识到语言的千差万别以前就早已在欧洲扎下了根。
(4)一个完整的句子
I found an old man lying on the ground and I took him to hospital in a taxi immediately, which was why I was late that morning.我发现一个老人躺在马路上，我立即乘出租车把他送到了医院，这就是那天上午我迟到的原因。
