If you had asked him yesterday, you would k now what to do now.
If it had rained last night (过去), it would be very cold today (现在).
虚拟条件句的从句部分如果含有were, should, 或had, 可将if省略，再把were, should或had 移到从句句首，实行倒装。
Were they here now, they could help us.
=If they were here now, they could help us.
Had you come earlier, you would have met him
=If you had come earlier, you would have met him.
Should it rain, the crops would be saved.
=Were it to rain, the crops would be saved.
If I were you, I would go to look for him.
If he were here, everything would be all right.
_____ to do the work, I should do it some other day.
A. If were I B. I were C. Were I D. Was I
C. 在虚拟条件状语中如果有were, should, had这三个词，通常将if省略，主语提前, 变成 were, should, had +主语的形式。但要注意，在虚拟条件状语从句中，省略连词的倒装形式的句首不能用动词的缩略形式。如我们可说 Were I not to do., 而不能说 Weren't I to do.
1） It is demanded / necessary / a pity + that…结构中的主语从句的谓语动词要用should 加动词原形， should 可省略。
It is (2)important that…+ (should) do
(3) a pity
(1)suggested, ordered, proposed, required, demanded, requested, insisted； + (should) do
(2)important, necessary, natural, strange
a pity, a shame, no wonder
(3)It is suggested that we (should) hold a meeting next week.
It is necessary that he (should) come to our meeting tomorrow.
order, suggest, propose, require, demand, request, insist, command, insist + (should) do
I suggest that we (should) hold a meeting next week.
He insisted that he (should ) be sent there.
注意： 如suggest, insist不表示"建议" 或"坚持要某人做某事时"，即它们用于其本意"暗示、表明"、"坚持认为"时，宾语从句用陈述语气。
The guard at gate insisted that everybody obey the rules.
（错） You p ale face suggests that you (should) be ill.
(对) Your pale face suggests that you are ill.
(错) I insisted that you ( should) be wrong.
(对) I insisted that you were wrong.
在suggestion, proposal, idea, plan, order, advice等名词后面的表语从句、同位语从句中要用虚拟语气，即（should）+动词原形。
My idea is that we (should) get more people to attend the conference.
I make a proposal that we (should) hold a meeting next week.