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。
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