2．表示与过去事实相反的条件
条件从句用动词的过去完成时，主句用should（第一人称）或would（全部人称）+have
done，也可用could, might代替should,
would。
If I had
known your telephone number yesterday, I would have telephoned you.如果昨天我知道你的电话号码，我就给你打电话了。
If it
had not rained this morning, I should have gone shopping.今天早上要是没下雨，我就去买东西了。
The
flood might have caused great damages to the people if we had not built so many
reservoirs.倘若我们没有修建这么多的水库，洪水就会使人民遭受巨大的损失。
3．表示在将来不太可能实现的条件
表示在将来不太可能实现的条件从句有三种形式：
①weredid
条件从句用动词的过去式（be动词用were），主句用should（第一人称）或would（全部人称）+动词原形。也可用could,
might代替should, would。
If he
were here tomorrow, I would speak to him.明天如果他在这里的话，我就和他谈谈。
If you
dropped the glass, it would break.你要是把杯子掉下来，会打碎的。
If she
had time, she could help me.她要是有时间，就会帮我了。
②should do
条件从句中不管什么人称都用should
do，可表示有偶然实现的可能性。
If it
should rain, the crops could be saved.假如下雨，庄稼可能就有救了。
We would
trust him if he should be honest.如果他真是诚实的，那我们就相信他。
③were to do
条件从句用were
+ to do。这种形式比较正式，常出现在书面语中，其假设成份很大，实现的可能性很小。
If I
were to do the work, I should do it in a different way.要是我做这项工作，我就会以不同的方式去做。
If the
headmaster were to come, what would we say to him假如校长来了，我们对他说什么呢？
对比：
将来不太可能实现的三种非真实条件句中的虚拟语气对比
If it
snowed tomorrow, I would stay at home.如果明天下雪的话，我就留在家里。
（常用形式）
If it
should snow tomorrow, I would stay at home.如果明天下雪的话，我就留在家里。
（可能性较小）
If it
were to snow tomorrow, I would stay at home.如果明天要下雪的话，我就留在家里。（可能性最小）
C．省略if的条件句
在书面语中，非真实条件句中有were,
had, should时，可以省略if，而把were,
had, should放在主语前，用倒装结构。
Were it
necessary, I might go without delay.如果需要的话，我可以立即去。（=
If it were necessary......）
Had you
taken my advice, you wouldn't have failed in the exam.你要是听了我的建议，就不会考试不及格了。（=
If you had taken my advice......）
Should I
have time, I would call on her.要是有时间，我就去看她。（=
If I should have time......）
D．错综条件句
虚拟条件句中的主句和从句涉及的动作发生的时间不一致，这时主句和从句的谓语形式应按照各自动作发生的实际时间来表达。
If you
hadn't watched TV yesterday, you wouldn't be so sleepy now.如果你昨天不看电视，你现在也就不会这么困了。（从句yesterday说明过去，主句now说明现在）
If they
had left home early this morning, they would arrive in half an hour.如果他们今天一早就离开家的话，再过半小时他们就该到了。（从句说明过去，主句说明将来。）
If you
hesitated this moment, you might suffer in the future.如果你此刻犹豫不决，你将来会吃苦头的。（从句说明现在，主句说明将来）
注意：
在很多情况下，我们也可以用was来代替当主语是第一人称或第三人称单数时的were。
If were
here tomorrow...
也可以说成：If
he was here tomorrow...
I wish I
were a bird.
也可以说成：I
sish I was a bird.
但倒装句型中的were不可被was来替换。
Were I
in your position,I would not do it.如果我处在你的位置，我是不会干这件事的。
E．含蓄条件句
非真实条件句中的条件从句有时不表达出来，只暗含在上下文中，这种句子叫做含蓄条件句。含蓄条件句大体有三种情况：
1．条件暗含在短语中
He would
not get such a result without your help.没有你的帮助，他就不会有今天的成果。(条件暗含在介词短语without
your help中)
But for
you, I could not be recovered so soon.要不是有你，我就不会恢复得这么快。(条件暗含在But
for you中)
This
same thing, happening in the past, would lead to a disaster. 同样的事，如发生在过去，就会酿成大祸。(条件暗含在分词短语happening
in the past中)
He must
have been there, or he never could know the place so well. 他一定是去过那儿，否则他绝不会对那个地方如此熟悉。(暗含条件是连词or)
2．条件可根据上下文推理出
It would
do you no good.这可能会对你没好处。(条件可能是if
you should give up the job如果你放弃这项工作的话)
They
could have won. 他们本来是会赢的。(条件可能是if
they had been patient如果他们有耐心的话)
Such
mistakes could have been avoided.这种错误完全能避免。（条件可能是if
you had been more careful如果你更加小心一点的话）
Why
didn't you tell me about it I should have helped you.为什么你不告诉我？我会帮助你的。（条件可能是if
you had told me about it如果你当时告诉我的话）
--- Did
you go to see him yesterday你昨天去看他了吗？
--- I
would have, but someone dropped over to my house for a visit.原想去的，但有人到我家来玩了。（条件是if
no one had dropped over to my house for a visit如果没人到我家来玩的话）
3．条件用其他形式来表示
She was
ill, otherwise she would have been present at the meeting.她病了，否则的话就会出席会议了。（条件通过连词otherwiseor表示出来If
she had not been ill...）
I told
him to go there himself, but perhaps I should have gone together with him.我让他自己去那里，但也许我应该和他一道去。（连词but暗示条件）
Suppose
you were in my shoes, what would you do假若你站在我的立场上，你会怎么办？（用supposesupposingproviding等词表示if）
To talk
with her, you would know she could not hear well.如果你同她交谈，你就会知道她的听力不好。（动词不定式带有假设的意思，相当于If
you should talk with her...）
Left to
himself, he could not have finished the work.要是放任他的话，他是不可能完成工作的。（动词的-ed形式表示条件If he
had been left to himself...）
提示：
在很多情况下，虚拟式已变成习惯说法，很难找出其暗含的条件。
You
wouldn't know. 你不会知道。
I would
like to go with you. 我愿意和你一起去。
三、虚拟语气在各种从句中的用法
虚拟语气除主要用于非真实条件句外，还可用在部分主语从句、宾语从句、表语从句、同位语从句、定语从句和状语从句之中。
A．名词性从句中的虚拟语气
主句中的动词、名词、形容词如表示要求、建议、命令或愿望等含义，与其相关的名词性从句（主语从句、宾语从句、表语从句和同位语从句）中的谓语动词要用虚拟语气。
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