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(huì)影響主句的意思,它與主句之間通常用逗號(hào)分開,例如：
This is the house which we bought last month.這是我們上個(gè)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(gè)月買的.（非限制性）
當(dāng)先行詞是專有名詞或物主代詞和指示代詞所修飾時(shí),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(gè)漂亮的花園.
This novel,which I have read three times,is very touching.這本小說很動(dòng)人,我已經(jīng)讀了三遍.
非限制性定語從句還能將整個(gè)主句作為先行詞,對(duì)其進(jìn)行修飾,這時(shí)從句謂語動(dòng)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(dǎo)非限制性定語從句.