在条件句种的用法。
条件句：条件句有两类，一类是真实条件句，一类是虚拟条件句。
1、真实条件句表示假设的情况是
有可能发生的，这种条件句中谓语用
陈述语气。
If he has
time, he
will go。
如果有时间，他会去的。
2、虚拟条件句表示纯然假设的情况或者
发生的可能性不大的情况。这种条件句中谓语要
用虚拟语气。
If he
had time, he
would go.
如果有时间，他会去的。
（一）表示与现在事实相反：
结构是：if
主语+动词过去式（be动词只用were），主语+could/should/might/would+动词原型+...
If you
arrived
a little earlier, you
could catch the train.
你要是早点到达，就为赶上火车了。（事实上来迟了）
（二）表示与过去事实相反：
结构：if
主语+had+过去分词，主语+could/should/might/would
have+动词原型+...
If you
had arrived a little earlier, you
could have caught the train.你要是早点到的话，就能赶上火车了。（事实上到达晚了，没赶上火车）
（三）表示与将来事实相反：
结构：if
+主语+一般过去时或should(were to) +
动词原形，主语+could/should/might/would+动词原型+...
If
he
were to miss the plane, he might come the day after
tomorrow.如果他误机的话，可能后天来。（误机的可能性不大）
|
条件状语从句
|
主
句
|
与过去事实相反
|
had + 过去分词
|
should /would/could/might + have + 过去分词
|
与现在事实相反
|
一般过去时(be用were)
|
would/should/could/might + 动词原形
|
与将来事实相反
|
一般过去时或should(were to) + 动词原形
|
Would/should/cold/might + 动词原形
|
四、虚拟条件句中if的省略
如果条件从句中含有动词had,should,were,可省略if,但这时要采用倒装句，把had,should,were放在主语之前。
<1>Were it to snow
tomorrow,they couldn't go out.
Were it to snow
tomorrow=If it were to snow tomorrow. 如果明天下雪，他们就不能出去了。
<2>The sports meeting would
be put off , should it rain tomorrow.
should it rain
tomorrow=if it
should rain tomorrow
如果明天下雨的话，运动会将推迟举行。
<3>Had you come here
yesterday evening,you might have met him.
Had you come...=if
you had come...
如果你昨天晚上来这儿子话，就有可能见到他了。
<4>Were it not for her help,I
couldn't have got over the diffficoulties.
Were it not for her
help=If it were not for her help
要不是她的帮助，这些困难我是无法克服的。
五、混合条件句中主、从句位于谓语动词时态的不一致
有时，条件从句所表示的动作与主句所表示的动作发生的时间不一致（如一个发生在过去，另一个发生在现在）。这时，我们要根据具体时间来调整动词的相应形式。
<1>If you
had taken
our teacher's advice, you
wouldn't make
such a mistake now.
如果你听取了老师的建议，现在就不会犯这种错误了。
<2>If you
were
in better health, we
would have
allowed
you to take part in the sports meeting.
如果你身体好一些，我们就会让你参加这次运动会了。
六、含蓄条件句中的虚拟语气
1.用
介词短语表示假设的情况，而不是以条件从句的形式表示出来，这时
主句中的谓语动词也要用虚拟形式。
〈1〉There
would be no life
on
the
earth
without air and water.
如果没有空气和水，地球上就不会有生命存在。
<2>But for the storm, we
should have arrived earlier.要是没有遇到暴雨，我们就会早到一些了。
But for the storm相当于If there hadn't been the storm
2.通过上下文或句子本身内在的时间概念把假设的情况表现出来。
<1>I was ill that day.
Otherwise I
would have taken part in
the sports
meeting.
那天我生了病，要不然就参加运动会了。
<2>It would be a mistake not
to help him.
不帮他将会是一个错误。
七、在某些从句中的用法
一、在主语从句中的用法
1.在“It is
/was+形容词/过去分词+主语从句”的句型结构中，主语从句的谓语动词要用“（should)+动词原形”形式，表示要求、建议、意愿、惊讶等。should可以省略，但不可换成would、could或might.
常用的形容词有：necessary, important,
natural, possible等。
常用的过去分词有:orderd, suggested,
required, decided, demanded, advised等.
<1>It is necessary that he
(should)be taken to the hospital at once.
<2>It is suggested that you (should) do morning
exercises every day.建议你每天做晨练。
2.在'It is /was a pity /a
shame/strange/no
wonder+主语从句'的句子结构中,主语从句的谓语动词要用'(should)+动词原形(或完成式)',表示惊奇、懊悔、失望、不满等。should可省略，但不可换成would,could或might.
<1>It is a pity that he
should live
at such a distance.他住的这么远，时为憾事.
<2>It is strange that
I
should not have been told.真奇怪,竟没有人告诉我。
二、在表语或同位语从句中的用法
在idea,
proposal, suggestion,
decision, advice, order等名词之后的表语从句或同位语从句中,谓语动词要用'(should)+动词原形
',表示要求、意愿、建议、命令等。should可以省略,但不可换成would, could, 或might.
<1>His suggertion is that we
(should) start
at once.(表语从句)
他的建议是我们应立即动身。
<2>He made a suggertion that we
(should) start at once.
(同位语从句)
他建议我们立即动身。
三、在宾语从句中的用法
1.虚拟语气用在动词wish后的宾语从句中,表示一种难以实现或过去没有实现的愿望。从句中的谓语动词要用过去完成时、过去时或过去将来时,不能用'should+动词原形
'或'should+完成时'形式。
<1>How I wish I had been able
to say the rules without a mistak!
我多么想能够一点不错地把那些规则说出来啊!
<2>I wish I remembered
his address.
我要是记得他的地址就好了。
2.用在would rather 后的宾语从句中,意为“真希望…,
宁愿…”。从句中的谓语动词有两种形式:
a. 用一般过去式表示将来。
<1>I'd
rather(that) you
went
home now.我愿意让你现在就回家。
<2>I say I'd rather (that) I
went
to work tomorrow
rather than
today.我说我希望明天去上班,而不是今天。
b.用过去完成时表示过去发生的动作
I'd rather (that) you
hadn't done
that.我真希望你当初没那么干。
3.用在insist,suggest等动词后的宾语从句中，表示建议、要求、命令、意志等。从句中的谓语动词要用“（should)+动词原形”，should可以省略，但不可换成would,could,或might.记住“I
drop caps.”这句话，会帮助你很快记住这些动词的（这句话是用这些动词的第一个字母组成的）。
I—insist, d—demand,
r—request/require,
o—order,
p—propose,
c—comand,
a—advise/ask(请求，要求）p—perfer,
s—suggest
<1>He insisted that we (should) discuss
the matter at the meeting.他坚持我们在会上讨论这件事。
<2>The boss ordered that all (should) take
part in the work.
老板命令所有的人都要参加这项工作。
<3>Our teacher suggest we (should) set off
right away.老师建议我们马上出发。
当suggest在句中的意思是“暗示表明”，而不是“建议”时，它后面的宾语从句中的谓语动词应用陈述语气，而不能用虚拟语气了。
例如： His pale face suggests that he is
in bad heslth..
他脸色苍白，说明他身体状况很差。
这句话中的is 不能换成was 或
were 。
四、在定语从句中的用法
用在“It is (high) time
(that)…'句型中，从句中谓语动词可以有两种形式，但都表示将来应该做的事。
1.谓语动词用过去式
<1>It is time we
left.
我们该走了。
<2>It is high time that you thought
about your future.你该考虑一下你的将来了。
2.谓语动词用should+动词原形，这时should不能省略。
<1>It is time we
should
leave.
<2>It is high time that you should
think about your future.
可以看出，在这种句型中谓语动词的两种形式是可以互换的。我们还可以把这两句话改写成含有不定式的句子：
It is time for us to leave.
It is high time for you to think about your
future.
五、在状语从句中的用法
1.用在as if /as
though引导的状语从句中，表示一种夸张的比喻和猜测。谓语动词的形式有三种情况，与虚拟语气用于wish后的宾语从句中谓语动词的形式相同，且be在主语为第一、三人称单数是可以用was或were.
a.与现在事实相反，用动词过去式
<1>It seems as if it were spring
alreaday.
好像已经是春天了。
<2>She loves the child as if she was (were)
his mother.她爱这个孩子，就象她是他的母亲一样。
b.与过去事实相反，用“had+过去分词”的虚拟结构
<1>They talked as if they
had been
friends for years 他们说话很亲热，就像多年的老朋友似的。
C.与将来事实相反，用would(might)+动词原形
<2>She cried as if her hear t
would
break.她伤心的哭着，好像她的心要碎了。
注：在look或其他个别动词（如：walk ,smell, sound,seem等）后由as if
(though)引导的状语从句中，可以用虚拟语气，也可以用陈述语气，具体用哪一种，应视具体的情境和意思而定。
<1>He looks as if he is
from
Guangdong.他看起来像是广东人。（事实上的确是广东人）
<2>He looks as if he
was
/were from Guangdong.（事实上不是广东人，只是长相上很像）
<3>He walks as if he is
drunk.(事实上的确喝醉了)
<4>He walks as if he
was
/were drunk.(事实上没醉)
<5>The foreigner speaks
Chinese so well as if he
was/were
a
Chinese.这个外国人的汉语说的这么好,就像他是一个中国人似的。
2.在以in order that, so that
引导的状语从句中,谓语动词多用'may(或might) +动词原形 '构成,在口语中也常用'can (或could)
+动词原形'。
<1>Let's hand in the exercises
earlier so that our teacher
may have time to correct
them.
我们早点交练习，好让老师有时间改。
<2>The teacher spoke slowly in order
that his students
could/might hear him clearly.
这位老师讲得很慢,以便他的学生能听清楚。
<3>She took a taxi so that she
could get there in time.她乘了一辆出租车，以便能按时到那儿。
在其他情况下的应用
一、用在口语中
虚拟语气用在口语中，使语气显得比较客气、委婉。这时谓语动词多由“should,
would, could, might+动词原形”构成。
1.陈述自己的看法
I don't think she
would be so
careless.我想她或许不至于这么粗心吧。
This is something I
should advise you not
to do. 我看你还是不这样做为好。
2.提出请求或邀请
Would you
mind turning down the radio? I'm
doing my homework. 把收音机音量关小一点好吗？我在做作业。
Could you
come
a little
earlier tomorrow? 明天你稍微来早一点好吗？
3.提出建议或劝告：
Perhaps we
could get
better
results this way. 这样做效果或许会好些。
It
wouldn't be
a good idea
for you to go there by bike. 骑自行车到那儿去不会是个好主意。
4.提出问题
Which bus
might I
take?
我能坐哪路车？
Could you
leave
your telephone
number with us?
你能把电话号码留下吗？
二、用在由if only引起的惊叹句中
虚拟语气用在由if only
引起的惊叹句中，表示一种愿望。这种愿望的实现是很渺茫的。根据表示的时间的不同，谓语动词有以下两种形式：
1.表示与现在的事实相反，用过去式
If only I knew
what he wanted!
要是现在我知道他想要什么就好了！
2.表示与过去的事实相反，用过去完成式
If only the train
had
arrived
there on time!
要是火车（那时）能准时到达那儿就好了！
三、用在谓语为表示希望、打算或意图的动词的句子中
英语中表示希望、打算或意图的动词有
hope ,think, expect, intend,mean, want 及suppose
等，它们的过去完成式可以用来表示一个未曾实现的想法，打算或意图，实际上也是虚拟语气的一种表达形式。
<1>They
had wanted
to
attend the meeting,but they could not go there for the bad
weather.
他们原本想参加会议，但因为天气不好未能去。
<2>I
had expected
to
see more of Beijing,but my train will leave at three this
afternoon.
我原本期待者多看看北京，但火车下午3点就要开了。
（资源来自网络，本人有修改！）
