题目内容：

this is the reason 【why】he was late for school? this is the reason 【that】he gave me yesterday
（he was late for school）不是一个完整的句子吗?应该填【that】呀! he gave【?】 me yesterday缺成分呀,为什么第二个空不填why?

优质解答

the reason why ...意为 什么 的原因；the reason that/which 后跟修饰reason的定语从句,比如the reason he gave us 意为 他给我们的理由.
如果表示的是“做什么事的原因”之类,就用why；如果是“他给的原因”之类,就用that/which.
体会一下下面两句话的含义.
this is the reason 【that】he gave me yesterday.这是他昨天给我的（某件事情的）原因.
this is the reason 【why】he gave me yesterday.这是他为什么昨天给我的原因.