一、在复合句中用作同位语的从句叫同位语从句.它一般跟在某些名词后面,用以说明该名词表示的具体内容.如：
I heard the news that our team had won．我听到了我们队获胜的消息.
I had no idea that you were here．我不知道你在这里.
二、可以跟同位语从句的名词通常有news,idea,fact,promise,question,doubt,thought,hope,message,suggestion,words（消息）,possibility等.如：
I’ve come from Mr wang with a message that he won’t be able to see you this afternoon． 我从王先生那里来,他让我告诉你他今天下午不能来看你了.
三、英语中引导同位语从句的词通有连词 that,whether,连接副词 how,when,where等.（注：if,which 不能引导同位语从句.）如：
l have no idea when he will be back．我不知道他什么时候回来.
He must answer the question whether he agrees to it or not.
他必须回答他是否同意这样一个问题.
四、有时同位语从句可以不紧跟在说明的名词后面,而被别的词隔开. 如：
Several years later,word came that Napoleon himself was coming to inspect them．
几年以后,有消息传来说拿破仑要亲自视 察他们.
The thought came to him that maybe the enemy had fled the city．
他突然想起可能敌人已经逃出城了.
五、同位语从句与定语从句的区别.
1、同位语从句与前面的名词是同位关系,即说明它前面名词的内容；而定语从句与前面的名词是修饰与被修饰关系,即限定它前面的名词范围,或补充一些情况.如：
The news that l have passed the exam is true．我通过了考试这一消息是真的.
（同位语从句,即从句所表达的意思就是前面名词的内容.）
The news that he told me just now is true．他刚才告诉我的消息是真的.
（定语从句,从句对前面名词起修饰限制作用,即“他告诉我的”那个消息,而不是别的消息.）
2、关系词在句中是否做成分 .如：
The idea that computers can recognize human voices surprises many people.
计算机能够识别人的声音的想法使许多人感到惊奇.（that在从句中不充当任何成份.）
The idea that he gave surprises many people．他提出的观点令许多人感到吃惊.
（that在从句中作gave的宾语.）
3、从句是否有疑问的意义.如：
eg.Do you remember the day when i told you that i loved you?(when引导的从句不表示疑问,所以这是一个定语从句.）
eg.I have asked the question why it was true just now.(why引导的从句表示疑问,所以这是一个同位语从句.）
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.
六、典型例题
例1：I have no idea when he will be back.
析：he will be back意义不完整,应加“什么时候”的含义才能表达idea的全部内容,因此应用when引导同位语从句.
例2：I have no impression how he went home,perhaps by bike.
析：he went home意义不完整,应加“如何”的含义才能表达impression的全部内容,因此应用how引导同位语从句.
例3：Information has been put forward ____ more middle school graduates will be admitted into universities.（NMET2001上海）
A.while B.that C.when D.as
析：答案为B.more middle school graduates will be admitted into universities是Information的内容,且Information不在从句中作成分,所以该句为同位语从句.应将该句区别于：
It is said that more middle school graduates will be admitted into universities,this is the information ____ has been put forward.
A.what B.that C.when D.as
析：答案为B.that has been put forward为information的修饰性定语,且information在从句中作主语,所以该句为定语从句.
例4：She heard a terrible noise,____ brought her heart into her mouth.（MET91）
A.it B.which C.this D.that
析：答案为B.分析语境含义、句子结构和句子成分可知,该句为非限制性定语从句,先行词为a terrible noise,且它在从句中作主语.应将该句区别于：
I can’t stand the terrible noise ____ she is crying loudly.
A.it B.which C.this D.that
析：答案为D.she is crying loudly是the terrible noise的内容,且the terrible noise不在从句中作成分,所以该句为同位语从句.