Question

the students didnt manage to work out the problem

The students did not manage to work out the problem-------their teacher had explained how.
A)when
b)until
c)unless
d）as
这个题我觉得应该是选B,可是却说不出不选C的原因,麻烦能否解释的清楚一些为什么

Answer 1

答案是B
你的句子是一般过去时.可以改写成：The students managed to work out the problem AFTER / WHEN their teacher had explained how.
如果用了 -- - unless -- - -结构,往往指的是条件状语从句.如果是条件状语从句就必须是“主将从现”了.所以,这里时态不符合.
看例句：I won't be able to work it out unless you tell me how to (work it out)-------这是条件状语从句------时态是主将从现.