Only when he had handed in his exam paper did he realize he had made several mistakes.

问题描述:

Only when he had handed in his exam paper did he realize he had made several mistakes.
为什么不用has he realized
has he realized 和did he realize的区别

he had handed 过去完成时态的使用就是要退两步,简单说,在一般过去时态的基础上再向前退一步,现在完成时态属于现在时态范畴,不能直接退到过去完成啊