Question

if引导虚拟语气三种用法，did,should do, were to do有什么区别？？

Answer 1

did 即一般过去时，表示与当前的实际情况相反的假设，
If you were not here , I would leave at once . 要不是你在这儿，我就立即走人了。
should do 表示与将来可能的情况相反， were to do 也是如此，表示与将来可能的情况相反。
If it should rain tomorrow , the plan would be ruined . 如果明天有雨，计划就泡汤了。

Answer 2

If it rained tomorrow,our picnic would be put off
=If it should rain tomorrow,our picnic would be put off
=If it were to rain tomorrow,our picnic would be put off
这三个意思相同，都表示对将来情况的虚拟
要注意的是
If it rained now,our picnic would be put off
rained还可以表示对现在的虚拟
所以did有两种可能性，
should do, were to do 只有一种可能性！

Answer 3

这个事对未来虚拟的三种不同表达方法！ if 主语+过去式/should do/ were to do
did 还可以表对现在的假设，should do 和 were to 侧重对未来的假设