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在主语从句表语从句同位语从句中只能用
1..位置不同：理论上讲,主语从句在主语的位置（当然实际中会有变式,如后面提到的头沉问题）,表语在表语的位置,其他同理
2..语法功能不同：这是关键.主语从句充当主语,（可能是直接形式主语.也可能是逻辑主语,因为形式上,主语可能有代词充当,如it）,表语从句起表语作用,其他同理
3..从句是相对于主句来说的,从句是套在主句里面的小橘子.在一个简单句里,每个语法成分基本都是一个词组充当的,如：It is obvious.（这是很显然的.）it 充当主语,is 系动词,obvious表语.
但是实际语言使用中,我们总想用一个橘子表达尽可能多的信息量,却远远不止是一个简单句可以完全囊括的.如句子：他是对的是很显然的（即：很显然,他是对的）,想表达充当主语的内容：“他是对的”,显然,一个词组或一个结构是不够的,为此就需要引进从句,按照语法规则,内嵌进主句中：如,That he is right is obvious.that he is right,充当主语成分,是个分句,所以,叫主语从句.当然,为了美感,也就是常说的,避免头沉,我们常说,It is obvious that he is right.其他从句,同理.
例：
1...在That he is right is obvious.中,主语从句是That he is right,直接的形式上的主语；变式--->>> 在 It is obvious that he is right.中,that he is right 是主语从句,是逻辑主语,即真正的句子主语,形式主语是It.
2...表语从句：What he is interested in is how to make money.表从是how to make money,作表语.
3...同位语从句：用来解释展开所修饰的同位语所包含的信息的从句.如：比较经典的：The news came that they won./ The news that they won came.
that they won 都是the news 的同位语从句,即the news 所包含的新闻内容就是他们赢了
4...宾语从句：最常见的：I think that you are right.从句 that you are right 做think的宾语,所以叫宾语从句.
表语从句和同位语从句的区别
首先，同位语从句、表语从句属于名词性从句，在语法上相当于一个名词。而定语从句属于形容词性从句，相当于形容词起到修饰限定作用。
1.同位语从句
顾名思义，从句跟它所对应的先行词是“同位”的，地位相同。一般由that引导。
例：I heard the news that our team had won.句中news即为先行词，that our team had won即为同位语从句。本质上，先行词就等同于同位语从句，同位语从句呢就是对先行词进行补充说明。
2.定语从句
定语的作用就是限定、修饰，这与形容词在用法上是一致的。
例：Those who are willing to attend the party , sign your name please.其中those为先行词，who为引导词， who are willing to attend the party 为定语从句。
小结：同位语从句跟定语从句的区别是把引导词去掉后是否为一完整的句子。如同位语从句that our team had won，把that去掉仍为完整的句子；定语从句who are willing to attend the party ，将who去掉后，句子就缺少了主语。
3.表语从句
表语从句就是用一个句子作为表语，说明主语是什么或者怎么样的。通常在系动词be seem, appear, prove, turn out, grow 之后。连接词可以由that, what, who, when, where, which, why,whether,how.whoever，whomever，whichever ，whatever担当。
例句：
His suggestion is that we should stay calm. The question is when he can arrive at the hotel.
其实表语从句比较简单，如果是系动词+一个句子，那么这个句子就叫做表语从句。
个人来看，同位语从句与定语从句比较容易混淆，还需多练习，多思考。
同位语从句和表语从句怎么区分
The problem is whether you could give us some valuable advice .（表）
2.His suggestion is that we should finish the work at once.（表）
3.My idea is that we should do it right now.（表）
4.The problem is that he can not speak English.（表）
这几个句子都是表语从句。关于同位语从句，请参考以下资料：
1. 同位语从句的意义
同位语从句用于表示名词的内容，对其加以阐明名解释。能跟有同位语从句的通常是具有一定内容含义的名词，如：fact（事实），idea（想法），news（消息），hope（希望），conclusion（结论），certainty（确实的事），evidence（证据），suggestion（建议），order （命令），problem（问题）等。例如：
The doctors came to the conclusion that the patient was suffering from cancer. 医生得出的结论是病人患了癌症。
The discovery that every heavenly body has gravity was made by Newton. 每个天体都有引力的发现是牛顿作出的。
2. 同位语从句的结构
同位语从句通常由that引导，但随着其先行词的不同，也可由whether, when, where, how, what, why 等引导。例如：
They are faced with the problem whether they should continue the work. 他们面临着是否要继续那项工作的问题
I have no idea when he will return. 我不知道他什么时间回来。
3. 同位语从句的位置
同位语从句一般情况下紧跟在有关的名间后面。然而，有时则不是这样，它被其他的词隔开了，或者说被后置了。例如：
An idea occurred to him that he might borrow the money from his father. 他突然想起他可以从父亲那儿借到这笔钱。
They heard the news on the radio yesterday that a typhoon was ocxning. 昨天他们从收音机里听到台风要来的消息。
表语从句和同位语从句的区别是什么
引导同位语从句的单词很多new，idea，fact，promise，question，doubt，thought，hope，message，suggestion，word（消息），possibility等；
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 if or not. 他必须回答他是否同意这样一个问题。
2.定义：
A 表语从句就是用一个句子作为表语。
The problem is puzzling.
主语 连系动词 形容词作表语
The problem is when we can get a pay rise.
主语 连系动词 一个句子作表语---表语从句
连接表语从句的连接词有：that, what, who, when, where, which, why,
whether,how.
He has become a teacher.
He has become what he wanted to be ten years ago.
She has remained there for an hour.
She has remained where I stood yesterday for an hour.
His suggestion is good.
His suggestion is that we should stay calm.
The question is confusing.
The question is when he can arrive at the hotel.
who will travel with me to Beijing tomorrow.
why he cried yesterday.
how I can persuade her to join us in the party.
whether the enemy is marching towards us.
二 注意：
A 表语从句一定要用陈述语序。
False: The question is when can he arrive at the hotel.
Right: The question is when he can arrive at the hotel.
B 不可以用if，而用whether 连接表语从句(as if 例外）。
False: The question is if the enemy is marching towards us.
Right: The question is whether the enemy is marching towards us.
Right: It looked as if he had understood this question.
C 不像宾语从句，在有表语从句的复合句中，主句时态和从句时态可以不一致。
Right: The question is who will travel with me to Beijing tomorrow.
Right: The question is why he cried yesterday.
D that在表语从句中不可以省掉。
表语从句只能置于主句之后，而主句的动词只能是联系动词。
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