关于 nonrestrictive clause 和 restrictive clause 区别

如题！

时间太长了，忘了，呵呵，哪位CDer帮忙解答一下：）

谢啦先！

引自薄冰语法

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.（限制性）

　　　The house, which we bought last month, is very nice.（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.

　　　My house, which I bought last year, has got a lovely garden.　

　　　This novel, which I have read three times, is very touching.

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.

　　　Liquid water changes to vapor, which is called evaporation.

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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