先行词是定语从句的引导词前面的名词.在定语从句中充当一定的成分.被定语从句修饰的名词、代词称为先行词.我们可以这样认为,之所以称它为先行词,是因为修饰它的定语从句总是放在它之后,先行词总是出现在定语从句之前,而不同于普通形容词修饰名词时,形容词位于名词之前.This is the book which I am looking for.中的book是先行词,which是引导词.He is Mr.Robertson who comes from England.中的Mr.Robertson.是先行词,who是引导词.简单的说,就是和修饰宾语的词或从句,词就是先行词 一个名词(或其它形式)对另一个名词或代词进行修饰,限定或说明,这个名词(或其它形式)就是同位语.同位语与被它限定的词的格要一致,并常常紧挨在一起.1.由两个或两个以上同一层次的语言单位组成的结构,其中前项与后项所指相同,句法功能也相同,后项是前项的同位语.Mr.Smith,our new teacher,is very kind to us.我们的新老师史密斯先生对我们很好.Mr.Smith是主词our new teacher的同位语,指同一人.Yesterday I met Tom,a friend of my brother's.昨天我遇到了我弟弟的朋友汤姆.a friend of my brother's是受词Tom的同位语,指同一人.2.如同位语与其同位成分关系紧密时不用逗点隔开;如同位语对其同位成分只作补充解释时可用逗点隔开.He himself told me that his brother John is a world-famous doctor.他本人对我讲,他的兄长约翰是一位世界闻名的医生.himself和John都是单一的字作同位语,与其同位成分之间不用逗点隔开.Yesterday I talked to my English teacher,Mr.James.昨天我与我的英语老师詹姆斯先生谈过了.同位语Mr.James补充解释my English teacher,同位语与其同位成分之间可用逗点隔开.3.同位语除表示其同位成分的全部意义外,还可以表示部分意义.We Chinese people are brave and hardworking.我们中国人民是勤劳勇敢的.He is interested in sports,especially ball games.他喜欢运动,特别是球类运动.同位从句一般跟在某些名词如answer(答案),hope(希望),fact(事实),belief (信仰),news(消息),idea(主意；观念),promise(承诺),information(信息),conclusion(结论),order(命令),suggestion(建议),problem(问题),thought (想法)等后面.例如：He always works hard even if he knows the fact that he is not in good health.His failure was due to the fact that he had not practised for a long time.I had no idea that you were here.She told us her hope that she would become a pianist.He made a promise that he would never come late.注意：that 既可引导同位语从句又可引导定语从句,其区别在于：同位语从句由连接词that引导,连接词that本身无意义,在同位语从句中不充当任何成分,不可省略,不可以用其他词替代；定语从句由关系代词that引导,关系代词that在从句中充当一定的成分,作宾语时可省略,由when,why,where引导的 同位语从句和定语从句的区别在于：同位语从句由连接副词只起连接作用,没有指代作用； 定语从句由关系副词引导,关系副词具有指代先行词的作用,常用一个介词加关系代词替换.例如:I will never forget the day when we first met.（同位语从句,when为连接词） That is the special day (which/that) I will never forget.（定语从句,which/that为关系副词） This is the house where we lived fifteen years ago.（同位语从句,where为连接词） This is the house which we sold fifteen years ago.（定语从句,where为关系副词） 注：each有同位语,every没有 例：they each went there yestable