表语从句(Predicative Clause)就是在复合句中作表语的名词性从句。说明主语是什么，由名词、形容词或相当于名词或形容词的词或短语充当表语。和连系动词一起构成谓语。一般结构是“主语+系动词+表语从句”。
一、表语从句的引导词
引导表语从句的词有：
从属连词：that, whether, as if, as though,
because, as;
关系代词：who, what, which, whom, whose,
whatever, whoever, whomever, whichever等;
关系副词：when, where, why, how, however,
whenever, wherever等;
1. 由从属连词that, whether, as if, as though, because, as引导
The fact is that he doesn’t really try. 事实是他没有做真正的努力。
The trouble is that I have lost his address. 麻烦的是我把他的地址丢了。
My suggestion is that we should tell him. 我的建议是我们应该告诉他。
The question is whether the film is worth seeing. 问题是这部电影是否值得看。
【注意】whether 可引导表语从句，但与之同义的if则不能引导表语从句。
It isn’t as if you were going away for ever. 又不是你离开不回来了。
Now it was as though she had known Millie for years. 现在好像她认识米莉已有好多年了似的。
It is not as though we were poor. 又不是我们家里穷。
It is because I love you too much. 那是因为我太爱你了。
That’s because you can’t appreciate music. 这是因为你不能欣赏音乐。
If I’m a bit sleepy, it’s because I was upall night. 如果我有点困，是因为一夜没睡。
It's because I passed a slip of paper for John to Helen in class. 这是因为我在上课中替约翰传纸条给海伦。
【注意】because 可引导表语从句，但与之同义的since, as, for等也不用于引导表语从句。
He looked just as he had looked ten years before. 他看起来还与十年前一样。
2. 由关系代词who, what, which, whom, whose, whatever等引导
You are not who I thought you were. 你已不是我过去所想像的人。
The problem is who we can get to replace her. 问题是我们能找到谁去替换她呢。
The question is who (m) we should trust. 问题是我们应当相信谁。
What I want to know is which road we should take. 我想知道的是我们应走哪条路。
That’s what I want to stress. 这是我想强调的。
That’s what we are here for. 我们来这里就为了这个。
He is no longer what he was. 他已经不是以前的他了。
3. 由关系副词when, where, why, how, however等引导
The problem is how we can find him. 问题是我们如何找到他。
That was when I was fifteen. 这是我15岁时发生的事。
That’s where I first met her. 那就是我第一次遇见她的地方。
That’s why he didn’t come. 这就是他没有来的缘故。
That’s where you are wrong. 这就是你不对的地方。
二、使用虚拟语气的表语从句
在表示建议、劝告、命令含义的名词后的表语从句，谓语动词需用“should+动词原形” 表示，should可省略。常见的词有：advice, suggestion, order, proposal, plan, idea等。
My suggestion is that we (should) start early tomorrow.
我的建议是我们明天一早就出发。
三、语序及连词that的省略问题
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.
引导表语从句的that通常不省略。
My idea is that we should do it right away. 我的意见是我们应该马上干。
The trouble is that he is ill. 糟糕的是他病了。
四、表语从句的基本用法
1. 表语从句只能置于主句之后，而主句的动词只能是连系动词。例如：
The problem is how we can get the things we need. 问题是我们怎样能弄到我们需要的东西。(how 在表语从句中充当方式状语)
The scissors are not what I need. 这把剪刀不是我所需要的。(what在表语从句中充当宾语)
What I told him was that I would find him a good play. 我告诉他的是我会给他找个好剧本。(what在主语从句中作直接宾语，that作为表语从句的引导词在该表语从句中不充当句子成分，不能省略)
That is what I want to tell you. 那就是我想要对你讲的。(what在表语从句中充当直接宾语)
That is why she failed to pass the exam. 那就是她考试不及格的原因。(why在表语从句中充当原因状语)
2. 下面是两个与“That is why...”形式相似的结构，它们与“That is why...”结构之间的关系要能够辨析清楚：
(1)“That is why...”与“That is the reason why...”同义，只不过从语法结构上讲，“That is the reason why...”中why引导的是―个定语从句，将其中的the reason去掉则与“That is why...”结构一样，例如：
That is (the reason) why I cannot agree. 这就是我不能同意的理由。
(2)“That is because...”句型中从属连词because引导的名词性从句在此作表语，这也是个常用句型，意为“这就是为什么……/因为……”。“That is because...”与“That is why...”之间的不同在于“That is because...”指原因或理由，“That is why...”则指由于各种原因所造成的后果，例如：
He did not see the film last night. That is because he had to help his little sister with her homework. 昨天晚上他没有去看电影，那是因为他得帮助他的妹妹做作业。(第一句话说明结果，第二句话说明原因)
He had seen the film before. That is why he did not see it last night. 他以前曾看过那部电影，因此他昨天晚上没有去看。(第一句话说明原因，第二句话说明结果)