18.4 限制性和非限制性定语从句
 
1) 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。






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