the reason that he gave for his being late for his being late for school was that he had to take

第一个that 是定语从句（修饰 the reason）的开始,第二个是表语从句 (在was 后面) 的开始.为什么不用why that?在reason 后面的定语从句，以 that 引导；如果以 why 引导，语法上也是准确的，但是句子成份，有些语法学家认为 why 引导的从句是同位语从句。因为它是reason的内容和解释。