Only when _______ home______ that he had left his bag in the taxi he had taken.
A.he returned; did he realizeB.did he return; did he realizeC.he returned; he realizedD.did he return; he realized在线课程A
考查倒装。Only + 状语从句，从句不倒装，主句的主谓要倒装，，故选A。