一、引导表语的有that,whether,what,who,when,where,because,why,as if,as though以及as。
1、that引导表语从句
结构：The fact/truth/chance/result/reason/cause+is that,其中that不做成分，可以省略。
2、whether引导表语从句
在表语从句中，需表示是否之意时，只能使用引导词whether而不能用if
3、连接代词（what,who等）和连接副词（when,where,how）引导的表语从句，语序应该为陈述句语序、非疑问句语序，而且这些连接代词和连接副词在句子都作成分。
The town is no longer what it used to be
4、because和why引导表语从句
主要区别在于because用于结果已于前述，需要补叙原因时，也即“先果后因”的倒叙法；why则用于原因已于前叙，需进一步说明其结果的情况，也即“先因后果”的倒叙法。
He didn't come to school yesterday that was because he was ill.
还有一些固定用法：（1）The reason why .....is that....(2)The cause is that....
5、as if/though引导表语从句
在look,seem,appear,feel,smell,sound,taste等系动词之后常可接有as if/though引导的表语
It seems as if it will to rain.
6、as引导表语从句
The main reasons are as follows.
