Not until I read your letter the true state of the affairs.
A. did I understand
B. had I understood
C. I understood
D. I had understood
网友回答
(答案→)A. did I understand
B. had I understood
C. I understood
D. I had understood
解析：考查倒装。否定结构Not until置于句首时，主句要用部分倒装，由此可以排除C、D两项；此外，understand这一动作和“read”几乎是同时发生的，都发生在过去的某个时间，因此用一般过去时态。故A项正确。