摘要：英语中，为了专门表示假设的意义和其他与事实相反的内容，特采用虚拟式来表示。下面，就由小编细细为大家道来。
►动词原形构成虚拟式
用动词原形表示的虚拟句，就是无论主语怎么变，动词一律保持原形不变，及以不变应万变。主要有以下几种情况：
一、用于表示命令、决定、建议等词语之后that-分句中
1. 动词：用在decide, decree, demand, insist, move, order, prefer, propose, recommend, request, require, suggest, vote等动词后的从句中。
The teacher ordered that all the books be sent to the office. 老师命令到所有书本要送到办公室。
She insisted that she go to the south in the summer vacation. 她坚持暑假去南方。
We propose that Li Hua take the chair. 我们建议李华当主席。
2. 形容词：用在advisable, appropriate, desirable, essential, fitting, imperative, important, important, possible, necessary等形容词后的从句中：
It is essential that we be healthy. 我们的健康很重要。
It is advisable that these regulations be abolished. 取消这些规则的说法是可行的。
It is necessary that we study English well. 我们将英语学好很必要。
3. 名词：用在decision，decree, demand, instruction, order, requirement等名词后的从句中：
Our teacher gives us an instruction that all the homework be done like that. 老师告诉我们所有作业都要那样做。
He gives me advice that I do the homework first. 他建议我先做家庭作业。
The demand that all students stand there isnt proper. 让学生站那儿的要求是不合理的。
注意：以上的用法均可以与should+动词原形和to do形式替换运用。如：
The teacher ordered that all the books be sent to the office.
=The teacher ordered that all the books should be sent to the office.
=The teacher ordered all the books to be sent to the office.
二、If、only等引导的从句中
If, though, whatever, lest, so long as等引导的分句，表示推测、让步、防备等含义。
If the man be found guilty, he shall have the right of appeal. 若是被发现有罪，这个男的将有申诉权。
Whether she be right or wrong, she will have our unswerving support. 不管她对错与否，我们都一如既往地支持她。
注：在非正式语体中，以上句子还可以用should+动词原形。
三、用于某些固定说法中：表示祝愿、诅咒、禁止等意义
God bless you! 上帝保佑！
So be it. 就这样吧！但愿如此。
Suffice it to say that 只需要说 就够了。
Far be it from 远不
Home is home, be it ever so homely. 家就是家，不管它是多么平凡。
英语中，虚拟语气作为一种表示与事实相反的结构，除了用动词原形外，还可以用过去式表示。那么，在哪些情况下，虚拟句需要用过去时来表示呢？
►动词过去式表示虚拟语气
一、用动词过去/过去完成时
1. It is time that ...
It is time that children got up. 孩子们该起床了。
It is high time you left for Beijing. 你该离开去北京了。
2. I would rather...
I would rather you had left yesterday. 我宁愿你昨天就离开了。
Frankly, I would rather he was absent. 坦白讲，我宁愿他没在场。
3. If only...
If only I had enough money. 要是我有足够的钱就好了。
If only I had listened to my mothers advice. 我要是之前听从妈妈的建议就好了。
4. as if/ though...
She ordered me as if she was my boss. 她支使我，好像是我老板似的。
She looked at me as though I was a monster. 她像看只怪物似的看着我。
5. I wish that...
I wish that I were a bird. 我希望自己是一只小鸟。
I wish that I hadnt said that. 我真希望自己没那么说过。
二、用情态助动词过去时
1. 用于条件句：
If I were you, I would not take part in this activity. 如果我是你，我不会参加这个活动。
If you should lend me the money, I should be much obliged. 如果你把这钱借给我，我会感激不尽的。
2. 用于含蓄条件句：
But for your help, I couldnt finish the task.没有你的帮助，我就完成不了这项任务。
He who should violate the law would be punished. 违反法律的人，都要受到惩罚。
3. 其他语境：
I was shocked that she should forget me so quickly. 我很震惊，她竟然这么快就忘记我了。
因此，由上可知，如果该句叙述内容与事实相反，或有假设意义，且出现以上如it is time that...结构时，需要用动词的过去时或情态助动词的过去时来表示。
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