关于英语从句的问题？

第三题：The reason why he failed in the exam is that he was too careless.
这句话的成分是：The reason （why he failed in the exam） is 【that he was too careless】.
（why he failed in the exam）是why引导的定语从句，用来修饰主语The reason；而【that he was too careless】是that引导的表语从句，作be动词is的表语。
题主可以这样理解，由于两个括号内的成分都是从句，那么这两个从句的引导词why和that互不干扰，不需要因为前一个定语从句的why，就把后面的从句改为because，更何况because仅仅是一个连词，并不能引导从句。
不知道我有没有表达清楚哈，满意请采纳，如果还有疑问欢迎追问~

3，之所以把because改为that是因为The reason(原因)不能因为(because), 原因只能是什么。你可以说: 做什么事(说这话)是因为…。本句如果把The reason删除，because就不用改了。即〉: Why he failed in the exam is because he was too careless.

这个，我不懂的。

结构2：名词或代词(主格) + doing
1)My mother cooking in the kitchen, I was doing my homework in my room. （表伴随）我妈妈正在厨房做饭，我在房间里做作业。