Not until they left school how much their teachers loved them and helped them.
A. that they realized
B. had they realized
C. when they would realize
D. did they realize
网友回答
A. that they realized
B. had they realized
C. when they would realize
(答案→)D. did they realize
解析：考查倒装句，not until+从句置于句首，主句用部分倒装，助动词/be动词/情态动词放在主语前面，BD都是倒装，因为他们先离开学校才意识到老师多么爱他们，所以是过去时。选D。