Not until they left school how much their teachers loved them and helped them.
A. they realized
B. had they realized
C. they would realize
D. did they realize
网友回答
A. they realized
B. had they realized
C. they would realize
(答案→)D. did they realize
解析：本题考查倒装结构。Not until放在句首，主句要发生主谓倒装，再根据时态，“他们意识到”是在“他们离开学校之后”，故用一般过去时。故选D。