虚拟语气在英语中应使用什么时态

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.