Only when class began that he had left his book at home.
A. will he realize
B. he did realize
C. did he realize
D. should he realize
网友回答
A. will he realize
B. he did realize
(答案→)C. did he realize
D. should he realize
解析: only+状语放于句首,应构成倒装句,主语前要加助动词,由时态看为一般过去时。