英文中一共有三种语气，分别是：陈述语气、祈使语气和虚拟语气。
一、含义
在表示假想的、虚假的、与事实相反的或难以实现的情况时，表示主观愿望或某种强烈的情感时；当说话人强调其所说的话是基于自己的主观想法，愿望，假想，猜测，怀疑或建议，而不是根据客观实际时。
二、句型
1)if引导的条件状语从句
①条件从句与现在事实相反
句型结构：if+主句+动词的过去式（be动词常用were）,主语+should/would/could/might+动词原形
例：If I had enough money, I would run a company of my own.
如果我有足够的钱，我会开一家自己的公司。
If I were you, I would work hard.
如果我是你，我会努力工作
②条件从句与过去事实相反
句型结构：If+主句+had+过去分词，主语+should/would/could/might have+动词现在完成
例：If you had come earlier, you could have seen the teacher.
如果你早点来，你就能见到那位老师了。
③条件从句与将来事实相反
句型结构：If+主语+动词过去式/were to+动词原形/should+动词原形，主语+should/would/could/might+动词原形
例：If I should win the lottery, I would buy a house.
如果我中了奖，我会买一套房子。
If she were to come here, how would you tell her about this?
如果她来这里，你会怎么告诉她这件事？
（注意：在主句中，should只用于第一人称I和we。）
2)错综时间条件句
当条件状语从句表示的行为和主句表示的行为在时间上不一致，主句及从句谓语动词的形式要根据具体的时间做出相应的调整。(分析从句和...事实相反，主句和...事实相反)
If we had started earlier,we would catch the first bus.(要是刚刚我们起早点，我们现在就能赶上车了吗)
3)省略if的条件句
如果条件句中含有had、were、should，有时可以省略if，将were,had,should放在主语前，变倒装。
Had the doctor come last night, the boy would have been saved.
4)含蓄条件句(替代if的条件句)
介词或介词短语"without(没有)/thank for(多亏)/but for(若不是)+名词结构"代替if条件句表示的虚拟语气。
①表示对过去的虚拟：情态动词 would/could/should/ought to + have done；
②表示对现在或将来的虚拟：情态动词 would/could/should/ought to + do。
But for your help,I could not sit here.
5)在表达建议、命令、要求等含义相关的名词性从句中
从句结构：主语+（should）+动词原形（此处should可以省略）
常见动词有：“一坚持”、“二命令”、“三建议”、“四要求”
一坚持：insist
二命令：order，command
三建议：advise，suggest，propose
四要求：demand，require，request，ask
一推荐:recommand
一决定:decide
例：He insisted that the new employees (should) be trained first.
他坚持要求新雇员先接受培训。
【注意：suggest,insist表示"暗示，表明"“坚持认为”时，不用虚拟语气，根据具体情况确认时态。】
例：The girl insisted that she hadn't stolen the electric bike.
那个女孩坚持说她没有偷电动车。
（如何判断是否使用虚拟语气，看从句中的动作是否真实发生，如果已发生了，用陈述语气，如果尚未发生，用虚拟语气。）
6)表示“惊讶、吃惊、必要性、重要性、自然等”的形容词
句型结构：It is+形容词或过去分词或特定的名词+that...的主语从句中
接这种结构的形容词有：astonishing, amazing, important, strange, necessary, natural, shocked, unusual, surprising, essential等
It is strange that he should not come here. 很奇怪，他竟然没有来。
7)wish+宾语从句(但愿...,要是...就好了)
与现在愿望不一致：主语＋过去时；
与过去愿望不一致：主语＋ had ＋过去分词/ could ＋ have＋过去分词；
与未来愿望不一致：主语＋ would / could ＋动词原形。如；
I wish I were you.(与现在愿望不一致)
I wish I had visited the White House when I was in the States.(与过去愿望不一致)
I wish I could meet you tomorrow at the party.(与将来愿望不一致)
8)If only(要是...就好了)引起的感叹句相当于“How I wish ＋宾语从句”
If only he could come! 他要能来就好了。(与将来不一致)
If only I had known the answer! 我要早知答案就好了。(与过去不一致)
9)as if /through引导的方式状语从句
与现在愿望不一致：主语＋过去时；
与过去愿望不一致：主语＋ had ＋过去分词/ could ＋ have＋过去分词；
与未来愿望不一致：主语＋ would / could ＋动词原形。
He talks as if he had been on the spot.
The situation seems as if it would be out of control.
10)would rather/had rather/would sooner引导的宾语从句
与过去事实相反:had done
与现在或将来事实相反:did(were)
I’d rather you posted the letter right away.
I’d rather you had returned the book yesterday.
11)It’s (high)time句型
that从句时应该为 should do或did
It’s time that you went / should go to school.
