虚拟条件从句中谓语动词用一般过去式，主句用“would/should/might/could+动词原形”形式，表示对现在不可能实现情况的假想。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。需要注意，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。
虚拟条件句往往指不能实现或纯假想的情况，可以对过去、现在或将来进行假想。虚拟语气的本质就是对根本不可能的情况发生假设。
例句：
（1）Were they here now, they could help us.=If they were here now, they could help us.
如果他们现在在这里，就能帮助我们了。
（2）Had you come earlier, you would have met him.=If you had come earlier, you would have met him.
如果你早点来，就能遇见他了。
（3）If I were you, I would go to look for him.
如果我是你，就会去找他。
（4）If he were here, everything would be all right.
如果他在这儿，一切都会好的。
扩展资料：
虚拟条件句分类：
1、对现在情况的虚拟
条件从句中谓语动词用一般过去式，主句用“would/should/might/could+动词原形”形式，表示对现在不可能实现情况的假想。
If I were you,I would choose to work in a small town.
如果我是你，我会选择在小城镇工作。（事实上我不是你）
2、对过去情况的虚拟
条件从句中谓语动词用过去完成式，主语用“would/could/should/might+have+动词的过去分词”形式。
If it had not been for your help yesterday,I could not have caught the bus.
如果昨天没有你的帮助，我不会赶上公交车。（事实是你昨天已经帮助了我，所以我赶上了车)
3、对将来情况的虚拟
在表示与将来事实相反的条件从句中，谓语动词可用should do,were to do或动词的过去式三种形式表示，但意义略有不同。should表示的可能性最大，动词过去式的可能性次之，were to do 的可能性最小。
If things were to be twice，all would be wise.
假如凡事都可以重新做一次，人人皆可成为聪明人。（事实是并不是所有的事都可以重新做一次)
参考资料来源：百度百科-虚拟条件句
请看典型考题：
__________
your
address，I
would
have
written
to
you.
A.
Did
I
know
B.
Were
I
to
know
C.
Had
I
known
D.
If
I
should
know
此题应选
C。这是虚拟语气的倒装形式。当虚拟条件句中含有
were，had，should，could等动词时，可以将if省略，然后将
were，had，should，could
等词移至句首：
1.
If
he
should
fail，he
would
kill
himself.
→Should
he
fail，he
would
kill
himself.
万一失败，他就会自杀。
2.
If
I
were
you，I
would
do
it
at
once.
→Were
I
you，I
would
do
it
at
once.
假若我是你，我就会马上做。
3.
If
I
could
do
it，I
would.
→Could
I
do
it，I
would.
要是我能做此事，我一定会做。
4.
If
he
had
seen
you
yesterday，he
would
haveasked
you
about
it.
→Had
he
seen
you
yesterday，he
would
haveasked
you
about
it.
他昨天要是看到了你，他就会问你这事了。
顺便说一句，以上移至句首的
had不一定是助动词，如果是实义动词也可倒装：
If
he
had
money，he
would
buy
a
car.
→Had
he
money，he
would
buy
a
car.
他要是有钱，他就会买一辆小车。
引用地址:
http://www.yygrammar.com/Article/200905/1208.html