定語從句有限制性和非限制性兩種.限制性定語從句是先行詞不可缺少的部分,去掉它主句意思往往不明確；非限制性定語從句是先行詞的附加說明,去掉了也不會影響主句的意思,它與主句之間通常用逗號分開,例如：
This is the house which we bought last month.這是我們上個月買的那幢房子.（限制性）
The house,which we bought last month,is very nice.這幢房子很漂亮,是我們上個月買的.（非限制性）
2) 當先行詞是專有名詞或物主代詞和指示代詞所修飾時,其后的定語從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,他曾經(jīng)是我的老師.
My house,which I bought last year,has got a lovely garden.我去年買的的那幢房子帶著個漂亮的花園.
This novel,which I have read three times,is very touching.這本小說很動人,我已經(jīng)讀了三遍.
3) 非限制性定語從句還能將整個主句作為先行詞,對其進行修飾,這時從句謂語動詞要用第三人稱單數(shù),例如：
He seems not to have grasped what I meant,which greatly upsets me.他似乎沒抓住我的意思,這使我心煩.
Liquid water changes to vapor,which is called evaporation.液態(tài)水變?yōu)檎羝?這就叫做蒸發(fā).
說明：關(guān)系代詞that和關(guān)系副詞why不能引導非限制性定語從句.
1）介詞后面的關(guān)系詞不能省略.
2）that前不能有介詞.
3） 某些在從句中充當時間,地點或原因狀語的"介詞+關(guān)系詞"結(jié)構(gòu)可以同關(guān)系副詞when 和where 互換.
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 引導的非限定性定語從句,as和which可代整個主句,相當于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.