高分请英语中一虚拟语气问题

if引导的虚拟语气,与现在/过去/将来事实相反时,主句用would should could might 加动词原形/would should could might加 have p.p./would should could might 加动词原形
这里主句中的would should could might是什么东西啊？怎么用的，什么时候该用哪个
谢谢

would should could might是情态动词,should 常用于第一人称would 用于任何人称．意思是会，could might表推测，意思是可能会

虚拟词，表示虚拟语气，在虚拟句中be的过去式一般为were,如：if i were you,i would accept his offer.
表示将来的情况时，主句中第一人称可用should，其他人称用would;从句中任何人称都用should,不可用would;如：if it should rain tomorrow,what would you do ?如果明天下雨，你怎么办？

虚拟语气

1） 概念
虚拟语气用来表示说话人的主观愿望或假想，所说的是一个条件，不一定是事实，或与事实相反。

2） 在条件句中的应用
条件句可分为两类，一类为真实条件句，一类为非真实条件句。非真实条件句表示的是假设的或实际可能性不大的情况，故采用虚拟语气。

一,虚拟条件句:
条件状语从句是非真实情况,在这种情况下要用虚拟语气.
1,条件从句与现在事实不一致,其句型为:
If 主语+过去时,主语+should(could, would, 或might)+动词原形,如:
If I were you, I would study hard.
If it rained, I would not be here now.
2,条件从句与过去事实不一致,句型为:
If 主语+had+过去分词,主语+should(could, would, 或might)+have+过去分词,如:
If the doctor had come last night, the boy would have saved.
If I had not studied hard, I would have failed in the exam last term.
3,条件从句与将来事实不一致,句型为:
+do,主语+should(could…)+ 原形 do
过去时(与现在事实条件句一样).