Only when class began that he had left his book at home.A. will he realizeB. he did reali

问题补充：

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

答案：

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。