首先分清什么是主语,表语和宾语.英语句子一般结构是主语+谓语+（表语）+宾语一般句子中对主语进行修饰解释说明的句子成分为主语从句,表语从句、宾语从句同理.1． 定义：用作同位语的从句叫做同位语从句.
2． 用法：同位语从句的先行词多为fact, news, idea, thought, question, reply, report, remark等,关联词多用从属连词that.如：
They were all very much worried over the fact that you were sick.
对你生病这件事,他们都很焦虑.
Where did you get the idea that I could not come?
你在哪儿听说我不能来?
注：同位语从句偶尔由从属连词whether引导.如：
I have no idea whether he’ll come or not. 我不知道他是否来.
连接代词who, which, what和连接副词where, when, why, how亦可引导同位语从句.
The question who should do the work requires consideration.
谁该干这项工作,这个问题需要考虑.
We haven’t yet settled the question where we are going to spend our summer vacation.
到哪儿去度暑假,这个问题我们还没有决定.
解释：1．that引导的同位语从句与定语从句之区别that引导的同位语从句
that引导的定语从句
句法功能上
that只起连接从句的作用,无意义.在从句中不充当句子成分.不可省.
that替代先行词在从句中不仅起连接作用,还充当一个句子成分.如在从句中作动词的宾语时可省略.
意义上
从句是被修饰名词的内容.
从句起限定作用,是定语
如：The news that Mr. Li will be our new English teacher is true. (同位语从句,that不可省.)
李先生将是我们的新英语老师这个消息是真的.
The news (that) he told me yesterday is true. (定语从句,that在从句中作told的宾语,可省.)
他昨天告诉我的消息是真的.
2．一些表示“建议、命令、要求”的名词后所跟的同位语从句中,谓语动词用虚拟语气should+动词原形表示.should可省.如：
This is our only request that this (should) be settled as soon as possible.
这就是我们唯一的请求：尽快解决这个问题.