定语从句有限制性和非限制性两种.限制性定语从句是先行词不可缺少的部分,去掉它主句意思往往不明确；非限制性定语从句是先行词的附加说明,去掉了也不会影响主句的意思,它与主句之间通常用逗号分开,例如：
This is the house which we bought last month.这是我们上个月买的那幢房子.（限制性）
The house,which we bought last month,is very nice.这幢房子很漂亮,是我们上个月买的.（非限制性）
2) 当先行词是专有名词或物主代词和指示代词所修饰时,其后的定语从句通常是非限制性的,例如：
Charles Smith,who was my former teacher,retired last year.查理·史密斯去年退休了,他曾经是我的老师.
My house,which I bought last year,has got a lovely garden.我去年买的的那幢房子带着个漂亮的花园.
This novel,which I have read three times,is very touching.这本小说很动人,我已经读了三遍.
3) 非限制性定语从句还能将整个主句作为先行词,对其进行修饰,这时从句谓语动词要用第三人称单数,例如：
He seems not to have grasped what I meant,which greatly upsets me.他似乎没抓住我的意思,这使我心烦.
Liquid water changes to vapor,which is called evaporation.液态水变为蒸汽,这就叫做蒸发.
说明：关系代词that和关系副词why不能引导非限制性定语从句.
1）介词后面的关系词不能省略.
2）that前不能有介词.
3） 某些在从句中充当时间,地点或原因状语的"介词+关系词"结构可以同关系副词when 和where 互换.
This is the house in which I lived two years ago.
This is the house where I lived two years ago.
Do you remember the day on which you joined our club?
Do you remember the day when you joined our club?
由as,which 引导的非限定性定语从句,as和which可代整个主句,相当于and this或and that.As一般放在句首,which在句中.
As we know,smoking is harmful to one's health.
The sun heats the earth,which is very important to us.
