Question

I didn't manage to do it ____ you had explained how. A until B unless C when D before D为什么不对？

Answer 1

I didn't manage to do it ____ you had explained how.
这句话的意思是：直到你解释了如何做，我才做到。
这里应该是考not... until（直到...才）的用法。答案应该是A
如果选D
before如果后接从句，动词应该是过去式，因为before后面的动作应该是瞬间动作不能延续。此句里you had explained how用的是过去完成时，是延续的一种状态。因此不对

Answer 2

D的话应该把you explained how改成you explained how
选A吧。

Answer 3

选B
before用于时间前后的关系