It/This/That is because……，该句型是because 引导的表语从句.
eg.He was sent to hospital. That was because he was ill.他被送到医院，是因为他生病了。
此外，that，why同样可以引导表语从句，但that在句中没有词义，且不能省略；在内容上，why 强调结果，because强调原因。
eg. ①The reason was that you don't trust her.原因是你不信任她。
②He was ill. That was why he was sent to hospital.他病了，所以被送到医院来。
补充：下面的句子是原因状语从句在强调句中的表达，与表语从句不同：
It was because the weather was so bad that we stayed at home.正是因为天气不好，我们才呆在家里。（强调部分是because the weather was so bad，原句是: We stayed at home because the weather was so bad. ）
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