Only when 1 was fourteen the importance of study．
A. I realized
B. did I realize
C. had I realized
D. I had realized
网友回答
A. I realized
B. did I realize
C. had I realized
D. I had realized
解析：
句意：只有当我14岁时，我才意识到学习的重要性。Only修饰状语when位于句首时需使用部分倒装，本句的时态应使用一般过去式。