在句子中充当表语的是一个从句时，这个从句就叫作表语从句。表语从句可以由连词、连接代词、连接副词和关系代词等引导，并且这些词不能省略。

1.连接表语从句的系动词

表语从句所表示的内容总是和主句的主语形成对等关系，其功能是说明或解释主语。除了be之外，可连接表语从句的系动词有look, seem, appear, sound, taste, feel, remain等。

【真题回放】As many people hit middle age, they often start to notice that their memory and mental clarity are not what they used to be.

解析　As many people hit middle age中, as引导伴随状语从句修饰主句。that their memory and mental clarity are not what they used to be中, that引导宾语从句作notice的宾语；what引导表语从句，系动词为are。

译文　当许多人到了中年时，他们经常开始发现自己的记忆力以及头脑清晰度大不如从前。

【例】It seems that no one knows what has happened in the park.（=No one seems to know what has happened in the park.）

解析　本句中that引导表语从句。

译文　似乎没有人知道在公园里发生了什么事。

2.表语从句的引导词

（1）连词that, whether（注意that不能省略, whether不可以用if替换）。【真题回放】Part of the problem is that the justices are not_____by an ethics code.

[A]advanced

[B]caught

[C]bound

[D]founded

解析　本句中that引导表语从句。空所在的语境为产生这样的问题部分原因在于“法官没有被_____道德规范”。bound“约束”符合题意。

译文　部分原因在于法官没有受到道德规范的约束。

（2）连接代词what, which, who, whoever等（从句为陈述语序）。

【例】That is what a study published from the University of California and Yale University in the Proceedings of the National Academy of Sciences, has_____.

[A]defended

[B]concluded

[C]withdrawn

[D]advised

解析　published from……of the National Academy of Sciences是后置定语修饰study; that为主语, is是系动词, is后面的整个部分作表语，也即主语补足语。整个句子的主干提取出来就是“That is……”，是一个主系表结构，后边整个表语从句充当句子的表语。根据简化的句子“That is what a study has_____”来解题。题目选择的动词是说明study怎样才有了上面what表示的内容。所以此题选择concluded（推断；得出结论）。

译文　该药物旨在通过阻断长期记忆存储所需的蛋白质的生成来阻止记忆的形成。

（3）连接副词when, where, how, why等（从句为陈述语序）。

【真题回放】When the court deals with social policy decisions, the law it shapes is inescapably political—which is why decisions split along ideological lines are so easily_____as unjust.

[A]dismissed(www.daowen.com)

[B]released

[C]ranked

[D]distorted

解析　主句为the law……is inescapably political。when引导时间状语从句；it shapes为省略引导词的定语从句，修饰the law, it指代the court; which引导非限定性定语从句，修饰主句，其中包含why引导的表语从句。由文中语境得知，该半句主要表达“这也就解释了为何背离思想路线的决策被看作是不公正的，从而被轻易的……”。结合语境及四个选项的意思，可知[A]最佳。

译文　当法院处理社会决策时，它适用的法律不可避免地具有政治性——这也就是为什么不同意识形态的决策容易被认为是不公正的而不予考虑。

（4）关系代词that, who, which等（从句为陈述语序代词不能省略）。

【真题回放】Justice Antonin Scalia, for example, appeared at political events. That kind of activity makes it less likely that the court’s decisions will be accepted as impartial judgments. Part of the problem is that the justices are not_____by an ethics code. At the very least, the court should make itself subject to the code of conduct that applies to the rest of the federal judiciary.

[A]advanced

[B]caught

[C]bound

[D]founded

解析　空格所在句为包含that引导的表语从句，从句为陈述语气，代词不能省略。本句指出法官参与政治活动这类行为的原因（part of the problem）：法官不被道德准则所_____。选项中能将ethics code（道德准则）与the justices（法官＜行为＞）联系起来的应该是[C]bound（约束），该词还可以表示“捆绑；装订；结合”。

译文　例如，法官安东宁·斯卡利亚现身政治活动。这种行为会使法院裁决被视为公正判决的可能性降低。问题的部分原因在于法官不受道德准则的约束。至少，法院应当让自身服从那些普遍适用于联邦司法系统其他部门的行为准则。

（5）一些从属连词和词组如because, as, as if, as though也可以引导表语从句。

【真题回放】That’s because happiness is linked to the kind of longer-term thinking_____for making investments for the future.

[A]sufficient

[B]famous

[C]perfect

[D]necessary

解析　该句是主系表结构，其中because引导表语从句。根据本句意为“因为快乐与对未来投资有_____长远考虑相联系。”要求填写形容词，我们要考虑其搭配与其修饰成分。空格处搭配介词for，并且修饰“长远考虑”。因此[D]选项最符合语境要求。

译文　这是因为快乐与未来投资所必需的长期思考有关。

3.当句子主语为reason时的表语从句

当句子主语是reason时，表语从句应当由that而不是because来引导，且that可省略；但可以用“it/this/that is because……”来表示原因。

【例】The reason is that modern man seems unable to redesign his institutions fast enough to accommodate the new demands, the new intelligence, the new abilities of segments of society which, heretofore, have not been taken seriously.

解析　本句的主干为The reason is that……，此处引导表语从句的that不能换成because。

译文　原因在于，现代人似乎不能尽快及时地修正其已形成的各种习俗以适应那些迄今尚未被认真对待的、来自社会各阶层的新的需求、新的智慧和新的才干。