1. If I were you, I should study English.
2. I would certainly go if I had time.
1. If you had taken my advice, you would not have failed in the test.
2. If I had left a little earlier, I would have caught the train.
③were + to do
1. If you came tomorrow, we would have the meeting.
2. If it were to rain tomorrow, the meeting would be put off.
如：If he comes, he will bring his violin.
The volleyball match will be put off if it___.
A. will rain
D. is rained
注意：1）在真实条件句中，主句不能用be going to表示将来，该用shall, will。
如： （错） If you leave now, you are never going to regret it.
（对） If you leave now, you will never regret it.
如：If they were here, they would help you.
如：If she had worked harder, she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday, he would have prevented me from going.
从句： were+不定式；主句：would+动词原形 should+动词原形
如：If you succeeded, everything would be all right.
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
如：If you had asked him yesterday, you would know what to do now. （从句与过去事实相反，主句与现在事实相反。）
If it had rained last night (过去), it would be very cold today (现在).
比较if only与only if：
only if表示“只有”；if only则表示“如果……就好了”。If only也可用于陈述语气。
如：I wake up only if the alarm clock rings. 只有闹钟响了，我才会醒。
If only the alarm clock had rung. 当时闹钟响了，就好了。
If only he comes early. 但愿他早点回来。
It is(high) time that It is(high) time that 后面的从句谓语动词要用过去式或用should加动词原形，但should不可省略。
如：It is time that the children went to bed.
It is high time that the children should go to bed.
didn't need to do表示：过去不必做某事，事实上也没做。
如：John went to the station with the car to meet Mary, so she didn't need to walk back home.
John went to the station with the car to meet Mary, so she needn't have walked back home.
There was plenty o ftime. She___.
A. mustn't have hurried
B. couldn't have hurried
C. must not hurry
D. needn't have hurried
答案：D。needn't havedone. 意为"本不必"，即已经做了某事，而时实际上不必要。 Mustn't have done用法不正确，对过去发生的事情进行否定性推断应为couldn't have done,“不可能已经”。must not do不可以（用于一般现在时）
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。
如：It is suggested that we(should) hold a meeting next week.
It is necessary that he(should) come to our meeting tomorrow.
2）在宾语从句中的应用在表示命令、建议、要求等一类动词后面的从句中。order, suggest, propose, require, demand, request, insist, command, insist+(should)do
如：I suggest that we(should) hold a meeting next week.
He insisted that he(should) be sent there.
如：The guard at gate insisted that everybody obey the rules.
判断改错：（错）You pale 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.
3）在表语从句，同位语从句中的应用在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。
如：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.
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。
如：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.
如：If I were you, I would go to look for him. 如果我是你，就会去找他。
If he were here, everything would be all right. 如果他在这儿，一切都会好的。
典型例题：_____to do the work, I should do it some other day.
A. If were I
B. I were
C. Were I
D. Was I
答案：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。