(从句)If + were/did(动词的过去式)，(主句)主语 + would/might/should/could + do(表示对现在情况的假设)
If I were you, I would not be so proud. 如果我是你，我不会如此自负。
If I were in your position, I would think better of it. 如果我处在你的位置，我会好好考虑它。
(从句)If + had done, (主句)主语+ would/might/should/could +have done(表示对过去或已经发生事情的虚拟假设)
What a pity it is that you didn't attend the concert yesterday! If you had attended the concert, you would have seen the famous singer.真遗憾昨天你没有去听音乐会。如果你去了，就能见到那位著名歌手。
Anyone in his position would have done the same.
If anyone had been in his position, he would have done the same. 任何处在他位置的人都会这样做的。
(从句)If + were/did(动词过去式)/were to do/should do，(主句)主语+ would/might/should/could + do(表示对将来的假设)
If he should refuse (= If he were to refuse=If he refused), it didn't matter at all. 万一他拒绝了，那也没关系。
If you shouldn't pass the college entrance examination, what would you do?万一高考不中，你该怎么办?
在虚拟条件句中，如果出现有were, had, should，可以省去if, 把这些词放在句子前面，构成虚拟倒装句。
Should he act like that again, he would be fined. 如果他还这样做，就要受罚。
Had the doctor come in time last night (=If the doctor had come in time last time), the boy would have been saved. 昨天晚上要是医生及时到达，小孩就会得救。
if only引起的感叹句，相当于 "How I wish + 宾语从句"，意思是"但愿……;要是……就好了"
If only he could come! 他要是能来就好了!
If only we students didn't have so much homework!要是没有这么多的作业该多好!
If only I hadn't been so careless in the exam! 我当时没有那么粗心就好了!
if it were not for… (= were it not for…)
if it hadn't been for… (= had it not been for…)"要不是因为有……;如果不是……"
[注意]这种结构中不能用否定结构的缩写形式，即不能用weren't it for…)
If it hadn't been for (= Had it not been for) the determined captain, all the passengers on board wouldn't have been saved. 要不是船长一直坚强，船上的旅客就不会得救。
If it were not for your rich parents, you couldn't live so easy a life. 要不是你父母有钱，你的生活不会如此安逸。
If it were not for the expense, I would go abroad now. 如果不是因为经费问题，我现在就出国了。
"but for + 名词"和"but that +从句"，意思是"倘若不是;要不是"，接虚拟语气
But for air and water, nothing could live. (= If there were no air or water, nothing could live.)如果没有空气和水，什么东西都难以生存。
But for the storm, we should have arrived earlier.
= If it hadn't been for the storm, we should have arrived earlier. 如果不是暴风雨，我们早就到了。
But for you, we couldn't have carried out the plan. 要不是你的话，我们无法实施那项计划。
She could not have believed it but that she saw it. 若非亲眼所见，她是不会相信的。
在动词insist(1坚持做某事)，order, command(2命令), advise, suggest, propose(3建议做某事)，demand, require, request, ask(4要求)等表示建议、命令、要求的名词性从句中谓语动词要用虚拟语气。基本句型：主语+ (should) + 动词原形。另外像decide, desire, intend, recommend等也要接(should)+动词原形结构。
advice, suggestion, order, demand, proposal, request, desire, command, decision, requirement等名词引导的同位语从句或表语从句，谓语动词用 (should) + 动词原形。
It's suggested/ advised/ demanded/ordered/requested/proposed/required/desired等结构后的主语从句中，谓语动词用(should) + 动词原形。
He insisted that he was innocent.=He insisted on his innocence. 他坚持说自己是无辜的。
He insisted that he had never done wrong. 他坚持说没有做错事情。
Are you suggesting that I'm not suited for the job? 你是在暗示说我不适合做那项工作?
The look on his face suggested that the teacher was quite satisfied with the result. 脸上的表情说明老师对结果感到满意。
It is necessary/important/natural/impossible/essential(基本的)等结构后的主语从句中要用虚拟语气，即主语+(should)+动词原形
It's necessary that Tom take the exam first. Tom有必要先参加考试。
With the society developing very fast, it's quite necessary/important that we (should) have a good knowledge of English and computer. 随着社会的快速发展，我们有必要精通英语和电脑。
It's strange/surprising/a pity/a shame/a surprise (that) … should do…should表示"竟然"
It's a pity that she should miss the chance. 很遗憾她错过了机会。
It's really surprising that a prophecy(预言)should coincide with the fact so exactly. 令人惊讶的是，预言和事实竟然如此巧合。
It's strange that he shouldn't pass the exam. 奇怪的是他竟然没有通过考试。