定语从句有先行词,且先行词在从句中充当成分.
例如：She is the girl who is good at maths.她就是数学好的那个女孩.有先行词 the girl ,先行词在从句who is good at maths中由who替代,作从句的主语.who被称为关系代词.
与之区别,同位语从句也有先行词,但从句是对先行词补充说明,先行词不在从句中作成分.
例如：We are discussing the question where she goes.where she goes是个同位语从句,解释说明the question,也就是说明具体是个什么问题.
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