同学你好，该知识点来自沪江网校《实用韩国语高级【8月班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
1. both of them指代的是复数，谓语应该是are，比如
Both of them are from poor families.
他俩都是穷苦出身。
2. had missed是过去完成时，指的是过去的过去，但这里只是说“听漏了”，不强调发生在另一个过去动作之前，所以用missed一般过去时。
3. 如果用A，前半句就是一个独立的句子，而逗号是不能连接两个独立的句子的。
所以要用as引导，让前半句变成定语从句。

as 引导非限制性定语从句，代替整个主句，对其进行说明。常用于as we all know, as is known to all, as is it, as is said above, as is often the case, as is reported in the newspaper等句式中，通常均由逗号将其与主句隔开。as 有“正如.....”,  “就像....”之意。
As we all know, the earth is round. 正如我们都知道的，地球是圆的。

可以在这里看看as引导的非限定性定语从句
http://www.hjenglish.com/new/p1209734/

祝同学学习进步~