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?
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?
只要记住这个reason why 后面加的一定是句子、而且内容一定是他要提的问题、he gave me yesterday 是来修饰reason的、that 特指前面的reason...
This is the reason why he was late for school.
这里why引导的是一个定语从句，the reason是先行词，关系副词why在定语从句中充当状语。why在这里等于for which.如果将这个定语从句分解，你就清楚了：
This is the reason. He was late for school for this reason.
第二句This is the reason that he gave me yesterday.
这里that引导的也是一个定语从句，关系代词that在定语从句中充当gave的宾语，这里不能用why.
This is the reason. He gave me the reason yesterday.
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.