同学你好，该知识点来自沪江网校《英语零基础直达大学六级【全额奖学金班】》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~
这两句的答案应该是：
I did not understand the problem until he had explained it.
We didn't disturb him until he had finished work.

until主句中的谓语动词为一般过去时，从句中的谓语动词为过去完成时，用来强调从句谓语动词的动作发生在主句谓语动词表示的动作之前。试比较：
Li Lei didn’t come until the film had begun. 直到电影开演，李雷才来。

实际上在使用中，为了避免这种时态上的麻烦，现在多都用一般过去时，比如
He did not stop until he reached the door. 他走到门边才停下。
She did not slow her stride until she was face to face with us.  她没有放慢脚步，径直走到我们面前。

祝同学学习进步~