Only then did he realize that he had lost a chance of achieving his dream 中为什么用did he realize
问题描述：
Only then did he realize that he had lost a chance of achieving his dream 中为什么用did he realize
答
Only + 状语（副词、 介词短语,从句） 放在句首时要用部分倒装,所以这么选
再如：
Only in this way 【can you learn】 English well.