i am afraid i have to go now i'd rather you didn't go alone.it is already too late

问题描述:

i am afraid i have to go now i'd rather you didn't go alone.it is already too late
谁能告诉我为什么要用did not,为什么不是do,为什么用过去式呢

这是 固定搭配
would rather sb did sth
好像还可用would rather sb should do sth