18.4 限制性和非限制性定語從句
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1) 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 that 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(dāng)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(jìn)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@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。
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