The reason why\thatThe reason ____ he was ill that day was actually made up by his brother这道题答案给的是that 说是同位语从句，我认为不对，顺便说一下reason后什么时候跟why什么时候跟that，
问题描述：
The reason why\that
The reason ____ he was ill that day was actually made up by his brother
这道题答案给的是that 说是同位语从句，我认为不对，顺便说一下reason后什么时候跟why什么时候跟that，
答
绝对是THAT
答
填why
这一句是定语从句
the reason是先行词
而从句he was ill that day非常完整.
所以用关系副词why
补充：若从句结构不完整,则用关系代词which或that
例如：The reason that/which he talked about is good.