真实条件句用于陈述语气，假设的情况可能发生，其中 if 是如果的意思。
句型： 条件从句 主句
一般现在时 shall/will + 动词原形
If he comes, he will bring his violin.
The volleyball match will be put off if it ___.
A. will rain B. rains C. rained D. is rained
1) 在真实条件句中，主句不能用be going to表示将来，该用shall, will.
(错) If you leave now, you are never going to regret it.
(对) If you leave now, you will never regret it.
2) 表示真理时，主句谓语动词便不用shall (will) +动词原形，而直接用一般现在时的动词形式。
句型 ： 条件从句 主句
一般过去时 should( would) +动词原形
If they were here, they would help you.
句型： 条件从句 主句
过去完成时 should(would) have+ 过去分词
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.
If he had come yesterday, I should / would have told him about it.
含义：He did not come yesterday, so I did not tell him about it.
If he had not been ill and missed many classes, he would have made greater progress.
含义： He was ill and missed many lessons, so he did not make greater progress.
句型： 条件从句 主句
一般过去时 should+ 动词原形
were+ 不定式 would + 动词原形
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.
If you had asked him yesterday, you would know 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.
在虚拟语气的从句中，动词 'be’的过去时态一律用"were"，不用was， 即在从句中be用were代替。
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
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.
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 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.
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.
从句动作先于主句动词动作 现在时 过去时
从句动作与主句动作同时发生 过去时 过去完成时
(had + 过去分词)
将来不大可能实现的愿望 将来时 would/could +
I wish I were as tall as you. 我希望和你一样高。
He wished he hadn’t said that. 他希望他没讲那样的话。
I wish it would rain tomorrow. 我希望明天下雨就好了。
2)Wish to do表达法。
Wish sb / sth to do
I wish to see the manager. = I want to see the manager.
I wish the manager to be informed at once. (= I want the manager to be informed at once.)
16.7 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。
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。
16.8 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。
It is time that the children went to bed.
It is high time that the children should go to bed.
16.9 need "不必做"和"本不该做"
didn’t need to do表示： 过去不必做某事, 事实上也没做。.
needn’t have done表示： 过去不必做某事, 但事实上做了。
John went to the station with the car to meet Mary, so she didn’t need to walk back home. 约翰开车去车站接玛丽，所以她不必步行回家了。
John went to the station with the car to meet Mary, so she needn’t have walked back home. 约翰开车去车站接玛丽，所以她本不必步行回家了。 (Mary步行回家，没有遇上John的车。)
There was plenty of time. 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 have done. 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)。