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限制性和非限制性定语从句
例如：
1)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。
