什么是表语从句（The Predicative Clause）？
在复合句中作表语的从句就是表语从句。
表语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的成分。
我们由此可得知：表语从句常出现在结构为“主语+系动词+表语从句”的复合句中。表语从句常用that, what, when, why, whether, how等来引导，引导词that常可省略。
This is what I want to know.
这是我想知道的。
The problem is how we will be able to finish writing the book report within three hours.
问题是我们将如何能够在三个小时之内写完读书报告。
His suggestion is that I should stay here overnight.
他的建议是我应该待在这里过夜。
That is why she was late for school this morning.
那就是她今天早上上学迟到的原因。
My question is when we can get to the airport.
我的问题是我们什么时候能到机场。
I really don't know whether you still love me.
我真不知道你是否依然爱我。
表语从句还可由because, as if (though) 等引导。
但主句的主语为reason, 只能用that引导表语从句，不能用because.
例如：Perhaps the most important thing to remember is that there is no one common type of life in America.
可能要记住的最重要的事情就是，在美国没有一种常见的生活方式。
在此句中，Perhaps the most important thing to remember 是主句的主语，说明的是一种原因（reason），所以表语从句要用that来引导。
再看下面的例句：
The reason why so many people died there is that there were not enough food supplies.
如此多的人死在那儿的原因就是没有足够的食物供应。
在此句中，The reason why so many people died there 是主句的主语，说明原因。there were not enough food supplies是表语从句，用that引导。（不能按照汉语表达习惯用because引导表语从句）。
It looks as if successful international cultural communication will make the world smaller.
看上去似乎成功的国际文化交流将使这个世界变得更小。
在此句中，look是系动词，successful international cultural communication will make the world smaller是表语从句，由as if 引导。
