虽然三者均可引导表语从句，但 that 没有词义，而why和because有自己的意思;另外，虽然why和because都可引导表语从句，但前者强调结果，后者强调原因。如：
The reason was that you don’t trust her. 原因是你不信任她。
The fact is that they are angry with each other. 事实是他们生彼此的气。
He was ill. That’s why he was sent to the hospital. 他病了，所以被送到医院来。
He was sent to the hospital. That’s because he was ill. 他被送到医院，是因为他生病了。