二、条件句中的虚拟语气
英语中条件从句有两类，一类是真实条件句，另一类是非真实条件句。如果假设的情况可能发生，是真实条件句；如果假设的情况是不存在的或不大可能发生的，则是虚拟条件句。
A．真实条件句
真实条件句表示的假设是可能发生或实现的，主句和从句的谓语动词都要用陈述语气。
If he
doesn't come at 8, we won't wait for him.如果他八点不来，我们就不等他了。
If a
flood happened in the past, there was usually a great loss of life and
property. 过去发生洪水的话，常有很大的生命和财产损失。
We shall
go there unless it rains tomorrow.如果明天不下雨，我们将去那里。
I'll let
you use my bike on condition that you keep it clean.如果你能保持车子干净，我就让你用我的自行车。
B．非真实条件句
在含有非真实条件句的复合句中，主句和从句的谓语动词都要用虚拟语气，其构成有三种形式：
与现在事实相反 if条件句的谓语：were did
主句的谓：would
(couldshouldmight) + do
与过去事实相反 if条件句的谓语：had
done
主句的谓：would(couldshouldmight)
+ have done
与将来事实相反 if条件句的谓语：didwere
主句的谓：should
do would (couldshouldmight) + do were to do
1．表示与现在事实相反的条件
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。可用情态动词could,
might代替should, would表示情态。
If it
were Sunday tomorrow, I would go and see my friends.明天要是星期天的话，我就去看望我的朋友。
If I
were you, I should go and try.我要是你，我就去试试。
If
wishes were horses, beggars might ride.
如果幻想能成为马匹，叫花子都有了坐骑
would go if they treated me like a slave.要是他们把我当奴隶对待，我就走。
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）
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