此题应填unless还是until
请看下面这道题：
I didn’t manage to do it __________ you had explained how.
A. unless B. until C. when D. since
此题应选B。这道题从句意来看似乎选A，B都行。不少考生看不出选项A错在什么地方，这里我们可以从句意和时态两方面来分析：
选A，句意为：你若没给我解释，我就不会做好它。—-按此选择，主句谓语应用would have managed之类的，也就是说这是一个虚拟语气的句子。
选B，句意为：直到你给我作了解释，我才会做。—-按此选择，你先给我解释，我后才会做，所以“解释”用过去完成时，“会做”用一般过去，顺理成章。
请做以下试题：
1. I didn’t know anything about it___ he told me.
A. unless B. until C. when D. since
2. _________ he spoke I hadn’t realized he was a foreigner.
A. Unless B. Until C. When D. Since
3. I didn’t realize he was our teacher _________ he came in front of me.
A. unless B. until C. when D. since
4. _________ he told me，I had no idea of it.
A. unless B. until C. when D. since
答案：1. B 2. B 3. B 4. B