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A: The reason why sb do sth 某人为什么要做某事原因

These are the reasons why we do this thing.

=These are the reasons that we do this thing.

这些就是我们做这事的理由.

B:The reason that sb do sth （有以下3种含义和理解）

(1)that具有关系副词why语法功能引导定语从句，在定语从句作状语

The reason that she did it is still a mystery.

=The reason why she did it is still a mystery.

她为什么做那件事仍是一个谜.

(2)that为连词，引导同位语从句

The reason that he was ill yesterday is such a bad excuse.

他昨天那个生病的理由真是个糟糕的借口.

(3)that为关系代词，引导定语从句，在从句充当，主语，宾语，表语 等成分。

 Did you hear the reason that he gave for being late? 

你听到他说的他迟到的原因没有.

The reason why sb do sth（表示为什么做某事的原因，属于定于从句,why为关系副词相当于for which在定语从句作原因状语）

That is the reason why Android tablets that do have the market are in fact, technically, phones.

这就是为什么从技术上讲，那些能够联入Android市场的平板电脑，本质上还是一部手机。

(备注，有时候关系副词可以用“具有关系副词语法功能that 关系词替换，并在从句作原因状语）

The reason that she did it is still a mystery.

=The reason why she did it is still a mystery.

她为什么做那件事仍是一个谜.

These are the reasons that we do this thing.

=These are the reasons why we do this thing.

这些就是我们做这事的理由.

The reason that I do so is clear.

=The reason why I do so is clear.

我为什么这样做的理由是清楚的.

The reason that sb do sth（表示具体做某事原因，即原因的具体内容，连词that不作任何成分，属于同位语从句=The reason is that sb do sth 表语从句）

He gave me the reason that he got up too late this morning.

他给了我他今天早上起床太晚的理由.

The reason that he was ill yesterday is such a bad excuse.

他昨天那个生病的理由真是个糟糕的借口.

(The reason is that he was ill yesterday 改成表语从句意思说得通）

The reason why he didn't go to school is that he was ill.（同位语从句）

=The reason that he didn't go to school is that he was ill.(定语从句）

他没有去上学的原因是他生病了。

the reason that 引导的同位语从句，可以改为连词that引导的表语从句意思说得通，就是同位语，否则不是同位语。

The reason is that he got up too late this morning.

原因/理由就是他今天早上起床太晚。