～主语和谓语动词是句子必不可少的主要成分，缺一不可。
～用句子作主语就是主语从句，从句作主语是简单主语的扩展。
～主语从句和同位语从句都属于名词性从句，所不同的是主语从句是句子的主要成分，同位语从句则是附加成分。
～同位语从句是对抽象概念 (名词) 进行具体说明的附加成分，缺少以后不会破坏句子的完整性，却影响意思的明确性。例如：
The fact (that you were not in) made us disapointed that day. 事实让我们那天都很扫兴。
His suggestion (that we should make a decision as soon as possible) has been adopted. 他提出的建议已经被采纳。