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虚拟语气
1 条件状语从句
2 与现在事实相反
3 与过去事实相反
4 与将来事实相反
5 条件句虚拟语气的省略和倒装
6 名词性从句中的虚拟语气
虚拟语气就是表示假设的情况，而不是表示客观存在的事实，甚至完全与事实相反。它是通过谓语动词的特殊形式来表示的。此外如需表达主观愿望或建议、猜测、命令等强烈的感情时，也可用虚拟语气。
1. 条件状语从句
虚拟语气表达的大多不是客观事实，而是一种假设，因此条件句是虚拟语气最为常见的载体。大家比较熟悉的是对可能发生的事情陈述时所使用的条件句，就是真实的条件句。比如If you buy that product, you will get one more free.“如果你买那个产品，可以免费再送一个。”If it rains next week, the show will be delayed.“如果下周下雨，表演将会被推迟。”这种情况下谓语动词使用原则是“主将从现”，即主句用将来时态，if从句用一般现在时。
虚拟语气所陈述的情况是假设，甚至与事实相反，不可能发生的事情，更多表达幻想或者遗憾的感情。例如：
If I were a male, I shall join the Chinese People’s Liberation Army. （事实上“我”是女性，也没有加入中国人民解放军。）
If I had been devoted to study in high school, I might have been to Beijing University. （事实是高中并没有投入精力去学习，现在也并未在北京大学读书。）
那么如何用谓语动词的特殊形式来表示虚拟语气呢？来看下面三种情况。
2. 与现在事实相反
如果所述情况与现在事实相反，从句：If+主语+动词一般过去式（be动词用were），主句：主语+ should/would/might/could+do。例如上述第一句，从句谓语动词were，而主语谓语从句为shall+join。
If there were no air or water, there would be no living things on the earth.
If he won the prize, the public would be more convinced.
3. 与过去事实相反
如果所述情况与过去事实相反，从句：If+主语+had+done，主句：主语+should/would/might/could+have done。
例如第1点的例句二“If I had been devoted to study in high school, I might have been to Beijing University”。
If he had taken my advice, he would not have made such a mistake.
4. 与将来事实相反
如果所述情况与过去事实相反，从句用过去时或者过去将来时：① if+主语+were to do,②if+主语+should+do, 或③if+主语+动词一般过去式（be动词用were）；主句：主语+should/would/might/could+do。例如：
If President Trump were to apologize for his absurd words, the world may forgive his deeds.
If the earth exploded, we could move to other planets.
5. 条件句虚拟语气的省略和倒装
(1) if条件句表达虚拟语气，还可以使用省略结构及倒装结构：去掉句首的If，将were, had, should置于主语之前即可。例如：
If I were a male, I shall join the Chinese People’s Liberation Army.
→ Were I a male, I shall join the Chinese People’s Liberation Army. （省略结构）
(2) 也可以用but for（要不是）, if only（要是，假如）等表示虚拟条件。
6. 名词性从句中的虚拟语气
名词性从句包括主语从句、宾语从句、表语从句和同位语从句，当这些从句表达愿望、建议、命令、请求等强烈情感时，主句常包含wish, hope, desire, demand, order, suggest, request等词以及它们的其他词性，从句谓语用should+do，should可以省略。例如：
It is suggested that man (should) have at least eight glasses of water each day.
The headteacher requires that all students (should) wear school uniforms.
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