When a ruler is partly in a glass of water, it looks as if it were broken.
叙述的情况是与事实相反的，as if 从句中应该使用虚拟语气。对一般事实的虚拟，用一般过去时。
I′d rather you went by train because the weather forecast said there would be heavy snow tomorrow.
would rather 后跟句子时，用虚拟语气。主语宁愿某人干某事。
用一般过去时，表示现在或将来做的事。
用过去完成时，表示过去要做的事。
Tom can take the radio apart and then put it back together again.I certainly wish he would teach me how.
wish 虚拟语气
wish+宾语从句表示不能实现的愿望。从句用虚拟语气。译为：可惜，就好了
表示现在不能实现的愿望，用动词过去式或were
表示过去不能实现的愿望，had+动词过去分词
表示将来不能实现的愿望，would + 动词过去分词
The board deemed it is urgent that these files should be printed right away.
董事会认为这些宗卷应该立刻打印。
urgent 虚拟语气标志词
urgent 在句中作形式宾语的补足语，其后的宾语从句应该使用虚拟语气，
即 should + 动词原形
某些词之后接虚拟语气，用should + 动词原形
1、 She put an extra blanket over the baby for fear that he should catch cold.
2、 I don′t think it is advisable that she deprive het little boy of his freedom to spend the spare time as he wish.
3、 It is recommended that the project shouldn′t be started until all the preparations have been made.
4、 In my eyes, his request that his cost be paid by the hour is completely reasonable.
1、 for fear that 以免，唯恐。后接虚拟语气 用： should + 动词原形
2、 It is + a. + that 从句中，主语从句中，谓语动词要用虚拟语气： （should）+ 动词原形，should 可以省略。
此类形容词有：imperative,desirable,vital,urgent,necessary,important等
3、 由“it is + 动词过去分词 + 主语从句 ” 引导的句式，常常表示要求、建议、提议、命令和愿望等语气，从句中谓语动词要用虚拟语气： （should ）+ 动词原形
这类词有： demand、 recommend、require、 insist、 order 、request 、 suggest 等
4、 order request demand proposal suggestion 等名词后面的同位语从句要用虚拟语气：（should）+ 动词原形
1、If there were no polysemy, Chinese would be much easier to learn.
如果没有一词多义，中文就容易学多了。
虚拟语气用于条件句中。
本题表示与现在事实相反：条件句用一般过去式 （were）主句用should / would + 动词原形
eg. If I were you, I would study hard.
表示与过去的事实相反：条件句用had+ 动词过去分词，主句用 should / would + have + 动词过去分词
eg. If he had taken my advice, he would have passed the test.
表示与将来的事实相反：条件句用 ①If + 主语+动词的过去式 ②If + 主语+were to+动词原形 ③If + 主语+should+动词原形 ，主句用should / would +动词原形
eg. If it were to rain tomorrow，they would not go to the cinema.
2、Everything would be very different if your father were still alive.
如果你父亲还在世，情况就不一样了。
从句用了过去式were 说明假设情况与现在或将来情况相反。应该用would+ 动词原形”
1、Had Jack known the truth, he would not have lent her any money.
2、There is a real possibility that these animals could be frightened, should there be a sudden loud nosie.
省略if 的虚拟语气
如果if从句中含有were/ should/ had时，则可以省略if，把这三个词置于句首，采用倒装语序。
eg.
If I were you，I would go to the party.
→Were I you，I would go to the party.
If it should happen，what would you do？
→Should it happen，what would you do?
If you had come earlier，you would catch the bus.
→Had you come earlier，you would catch the bus.
It is (high) time +从句 意为：到了该做什么的时间了。
后面的从句用虚拟语气, 动词用过去式，或者 should + 动词原形，should 不能省略。
eg.
It is time that the children went to bed.
It is high time that the children should go to bed.
到了该上床睡觉的时间了。
was / were to have done sth.本来要做成…的
I was to have finished the writing if I had not been called away.
如果没有被人叫走的话，我本来是要完成这篇写作的。
虚拟语气用法，was / were to have down 表示：本来要做成…的，表示与过去事实相反，从句谓语用 had / hadn't done
if only 虚拟语气，表示感叹性的愿望，意为：要是…就好了！
If only the committee would approve the regulations and put them into effect as soon as possible.
如果委员会会批准这些规定并尽快付诸实践就好了。
表示对将来的希望时，句中谓语动词用 would / could / might / should + 动词原形
本句 as soon as possible 表示该句是对将来的希望。
