英语名词性从句（四）：表语从句
概念
表语从句又是主语补足语从句，表语从句是位于系动词之后的句子，即主系表结构。实际上，主系表结构就是主语+系动词+主语补足语。在中有详细说明主语补足语的概念。
\[\frac{The\ secret}{主语} \frac{is}{系动词} \frac{that\ the\ rabbit\ ate\ a\ carrot}{主语补足语/表语}. \]上面的例句可以改写成主语从句：
\[\frac{The\ rabbit\ ate\ a\ carrot}{主语从句} \frac{is}{系动词} \frac{the\ secret}{表语}. \]引导词
"That" 引导表语从句，标识这个从句作主句的表语，这样的词叫连词。表语从句的连词有从属连词、连接代词、连接副词。
| 连词类型 | 连词 |
|---|---|
| 从属连词 | that、whether |
| 连接代词 | who、which、what、whom、whose、whatever、whichever、whoever |
| 连接副词 | when、where、why、how、whenever、wherever |
例句
The problem is that the rabbit is hungry.
The reason is that there's no more carrot.
The question is whether the rabbit is hungry.
The question is waht the rabbit should eat.
The question is where the rabbit should eat the carrot.