1. 虚拟现在时表示与现在事实相反的假设，其if 从句的谓语形式用动词的过去式(be 一般用were)，主句用would/ should/ could/ might +动词原形。If places were a like, there would be little need for geographers.如果各个地方都一样，就不需要地理学家了。
2. 虚拟过去时是表示与过去事实相反的假设，if 从句的谓语形式用过去完成时即had＋过去分词，主句用would / should / could / might + have +过去分词。If he had known your address yesterday,he would have telephoned you.
3. 虚拟将来时是表示对将来实现的可能性很小的或不确定的假设。If从句的谓语形式用一般过去式或用were to / should +动词原形，主句用would / should/could/ might +动词原形。If he were to leave today, he would get there by Friday
4. 省略if 采用倒装语序的条件句。有时可以把含有助动词、情态动词、be或have的虚拟条件句中的连词if省去，而将had , should, were 等词提到主语之前，即用倒装结构。Had he worked harder, he would have got through the exams.Were he to leave today, he would get there by Friday.Were I in your place, I wouldn’t do that.
5. 有时虚拟条件句并没来if 从句表示出来，而是用介词短语(otherwise, or, without, but for)、上下文或其它方式来表示。We didn't know his telephone number; otherwise we would have telephoned him.Without you help, I wouldn’t have achieved so much.But for your help, I would not have succeeded.
1. 在建议,要求,命令,想法后的动词后的从句里, 用should + 动词原形, should 可以省略. 如 advise, agree, command, decide, demand, determine, grant, indicate, insist, order, prefer, propose, request, require, stipulate, suggest, urge, vote.常考到的是: suggest, advise, demand, require, propose, insist, order, request.
His doctor suggested that he (should) take short leave of absence.
The author proposed that TV (should) be turned off at least one hour every day.
2. 在建议,要求,命令,想法后的名词后的从句里, 用should + 动词原形, should可以省略. 如 advice, agreement, command, decision, decree, demand, determination, indication, insistence, order, preference, proposal, request, requirement, stipulation etc.
It was Bill’s suggestion that everyone (should) have a map.
His suggestion was that everyone (should) have a map.
He gave us a suggestion that everyone (should) have a map.
3. 在It is/was +形容词后的that 从句中用should的结构, should 可以省略. 这类形容词常见的有: advisable, anxious, compulsory, crucial, desirable, eager, essential, fitting, imperative(绝对必要), impossible, improper, important, natural, necessary, obligatory, possible, preferable, probable, recommended, urgent, vital etc.
It‘s natural that she (should) do so.
It is essential that we (should) tell her the news.
4. 在 lest 和 for fear that(以免),in case(以防)从句中用should, should 可以省略.
She walked quietly lest she (should) wake up her roommates.
Have your gun ready in case we should need it.
III: wish 后的 that 从句中:
I wish I knew his address.
I wish I were young.
2. 表示过去没有实现或不可能实现的愿望, 从句用过去完成式或would, could, might + have + 过去分词.
I wish you had written to him.
I wish I could have slept longer this morning, but I had to get up and come to class.
3. 如果将wish改成wished, 其后that 从句中的动词形式不变.
4. 如果that 从句中用would , 一般表示 对现状不满或希望未来有所改变或请求
I wish he would answer my letter.
I wish prices would come down.
I wish you would help me.
I wish you would stop asking silly questions.
IV: 在 It’s about/ high/ first(second, third etc)time 后的that 从句中,用过去式.表示“该是做什么的时候了”
It is about time you were in bed.
It is high time we left.
It is the first time I came here.
V: 在would rather, would sooner, would just as soon 后的that从句中,用过去式或过去完成式,表示”宁愿做什么”
I would rather he came tomorrow than today.
John would rather that she had not gone to the party yesterday evening.
VI: 在if only 感叹句中,谓语动词与wish宾语从句的虚拟形式相同.
If only he didn’t drive so fast!(现在)
If only she had asked someone’s advice.(过去)
If only the rain would stop.(将来)
VII: 在as if / as though 从句中,表示和现在事实相反或对现在情况有所怀疑,用过去式;表示过去想象中的动作或情况, 用过去完成式.
He speaks as if he were on the spot.
She spoke to me as if I were deaf.
This device operated as though it had been repaired.
注: 1. 在 as if / as though 句中, 如果有可能成为事实, 用陈述语气.
He looks as if he is going to be ill.
2. 在 insist 后的从句中, 如果是坚持自己, 用陈述语气, 坚持别人做什么事情, 用虚拟语气.
She insists that she is right.
She insisted that I should finish the work at once.