I did not understand the problem. He explained it.请用until把两句连在一起

有两个动作，我理解和他解释
根据先后顺序，他先解释，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.

I did not understand the problem unil he explained it.

I did not understand the problem until he explained it to me.

I understand the problem until he explained it,.