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
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
C
"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。