Question

【题目】The students didn't understand the question until it (explain) again.

Answer 1

【答案】was explained
【解析】句意：学生们不理解这个问题直到它再次被解释了一次。It指的是the question，the question与explain之间是被动关系，根据didn't understand用的是一般过去时，所以这里应该用一般过去时的被动语态。所以答案是：was explained。