i had not understood the problem until he explained it.

问题描述:

i had not understood the problem until he explained it.
我再理解,过去完成式不应该是 had exlained

直到他解释后我才理解了这个问题.
这个主要讲的是not……until这个句型,“直到……才”
意思是:我先理解,没有理解.后来他给我作了解释.