虚拟语气在英语中应使用什么时态有些虚拟语气用的是一般过去时态,有的是一般现在时,有的又是现在完成时,请问有什么规则吗

虚拟语气用在一般现在时,
从句 IF + S + 一般过去时 ,主句 S +SHOULD/WOULD + V
例子：If I stayed in that school,I would pass the exam.
虚拟语气用在一般过去时,
从句 IF + S + 过去完成时,主句 S + SHOULD/WOULD have done
例子：If you had been here yesterday,you would have met him.
虚拟语气用在一般将来时,
从句 IF + S + was/were going to do ,主句 S +SHOULD/WOULD + V
例子:if it was going to rain,we wouldn't go on a picnic this Sunday.