第1句为什么关系代词用why,第2句中为什么关系代词用which,第3句中为什么关系代词用that/which1.that was the reason (why) mac refused to speak at the meeting2.that was the reason (which)mac refused to speak for at the meeting3.that was the reason (that/which)mac gave me the other day

这三句都是定语从句 1. why代替for the reason在从句中作原因状语。因为从句是个完整的句子。 2. which代替the reason在从句中做for的宾语。因为speak后出现了for，所以只能用which。 3. that和which代替the reason在从句中作give的宾语，因为give后没介词，所以能用that和which。
比如：This is the house. I was born in the house就能改成1. This is the house where I was born. 2. This is the house in which I was born. 3. This is the house that/which/省略 I was born in.

第一句先行词是reason，why用来修饰原因，说的是：这个原因就是为什么。。。
第二句refused to speak for sth,这个sth就要用which来代，作名词
第三句 gave me sth，这里缺少的也是名词，是宾语，用that/which

1.同位语从句,用why引导表示……原因
2.宾语从句,先行词在从句里位于for后充当宾语
3宾语从句,先行词在从句里充当宾语

第一句中与第二句中why和for which充当相同的成分解释原因
第三句中that和which充当gave的宾语