if you ( )遗忘you homework at home ,what would you do?

题目

if you ( )遗忘you homework at home ,what would you do?
( )填left,请问像这种“主将从现”的从句中填过去式的情况有那些?

答案

if的虚拟语气:
if+did(now)sth,sb would do.
if+had done(past)sth,sb would have done.
if+did/should do,sb would do.