I did not understand the problem.He explained it.请用until把两句连在一起
问题描述：
I did not understand the problem.He explained it.请用until把两句连在一起
可是书上的原句是这样的：I had not understood the problem until he explained it.为什么是had not understood 呢
答
有两个动作,我理解和他解释
根据先后顺序,我再理解,
或者,按照not until来翻译,
就是,直到他解释了,我才理解
所以,就是你的答案
I had not understood the problem until he explained it.