与现在事实相反 would/情态动词过去式+do were (不分人称)/did
与过去事实相反 would/情态动词过去式+have done had done
与将来事实相反 would/情态动词过去式+do should do / were to do
如：If the doctor had been available, the child could not have died.
There is a real possibility that these animals could be frightened, should there be a sudden loud noise.
Had it not been for the timely investment from the general public, our company would not be so thriving as it is.
Had Paul received six more votes in the last election, he would be our chairman now.
Your math instructor would have been happy to give you a make-up examination had you gone and explained that your parents were ill at the time. (句子前半部分为假设情况，而“父母病了”是事实)
I would have gone to visit him in the hospital had it been at all possible, but I was fully occupied the whole of last week. (前半部分为假设，后半部分是事实)
desire, advise, recommend, command, direct, order, ask, demand, request, require, insist, maintain, move, propose, prefer, urge, vote。如：
In the past men generally preferred that their wives work in the home.
I move that he be discharged for his serious mistake.
advisable, desirable, insistent, preferable, urgent, appropriate, compulsory, crucial, essential, imperative, important, necessary, obligatory, possible, probable, proper, vital, advised, arranged, commanded, demanded, desired, ordered, proposed, recommended, requested, required, suggested。如：
The board deem it urgent that these files should be printed right away.
It is essential that all these figures be checked twice.
insistence, preference, recommendation, suggestion, proposal, motion, desire, requirement, request, order, necessity, importance, regulation, rule, resolution, understanding。如：
John Wagner’s most enduring contribution to the study of Afro-American poetry is his insistence that it be analyzed in a religious, as well as worldly, frame of reference.
They keep telling us it is of utmost importance that our representative be sent to the conference on schedule.
(1)连词but, but that, or, or else；副词otherwise, unfortunately等表示转折假设。如：
A safety analysis would have identified the target as a potential danger. Unfortunately, it was never done.
Victor obviously doesn’t know what’s happened; otherwise he wouldn’t have made such a stupid remark.
(2)介词短语暗含假设条件，常用的有：without, but for, under more favorable conditions等。如：
But for the English examination I would have gone to the concert last Sunday.
(3)intended/meant/hoped/wished/plann或was/were +不定式完成式或had intended/meant/planned/hoped/wished
+不定式一般式暗示虚拟语气。如：I intended to have called on you, but I was busy at that time.
(4)情态动词完成式暗示虚拟语气。如：I should have called to make an airline reservation, but I didn’t.
would rather would as soon as though suppose…
had rather would sooner as if supposing…
If only… It is (high) time that…(从句中动词只用过去式)
如：His wife would rather they didn’t talk about the matter any more.
I’d rather you went by train, because I can’t bear the idea of your being in an airplane in such bad weather.
If I were in a movie, then it would be about time that I buried my head in my hands for a cry.
My father always talks as though he were addressing a public meeting.
(2)If it were not for… (与现在事实相反)
If it had not been for… (与过去事实相反) 相当于but for。如：
If it had not been for his help (= but for his help), we would not have succeeded.
If only the committee would approve the regulations and put them into effect as soon as possible.
(4)lest/for fear that/in case 从句谓语用(should+)动词原形。如：
The mad man was put in the soft-padded cell lest he injure himself.
Church as we use the word refers to all religious institutions, be they Christian, Islamic, Buddhist, Jewish, and so on.
The business of each day, be it selling goods or shipping them, went quite smoothly.