（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 isthe house which we bought last month.这是我们上个月买的那幢房子。（限制性）·Thehouse, 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：·CharlesSmith, 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。·Myhouse, which I bought last year, has got a lovely garden.我去年买的那幢房子带着个漂亮的花园。· Thisnovel, 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：·Heseems 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。·Liquidwater 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。