1. reason

reason后可接why／that引导的定语从句;

The _______ _____ _____ _____ _____was that he didn’t care about health．

1) 他生病的原因是他不注意自己的健康。

reason why he fell ill

句型: The reason why…is that...

我迟到的原因是我错过了早班车！ The reason why I was late（定语从句） is that I missed the first bus.