1、一想要(desire) 一宁愿(prefer) 一坚持(insist)二命令(order , command) 三建议(advise , suggest , propose/recommend) 四要求(demand , require , request , ask)中，无论主句谓语动词为何种时态，从句的谓语动词都用："should + do"。should可以省略。
eg: He suggested that we (should) take the teacher's advice.
He insisted that we (should) take the teacher's advice.
He demand that we (should) take the teacher's advice.
He ordered that we (should) take the teacher's advice.
insist意为“坚持某种动作”才用虚拟语气;意为“坚持某种观点，某个事实”则不用虚拟语气。
eg: He insists he is a student.他坚持说他是个学生。
这个语句表示的是事实，因此在这个语句中不能使用虚拟语气。
suggest意为“建议”才用虚拟语气，意为“暗示”则不用虚拟语气。
eg: His face suggests that he looks worried. 他的表情暗含着他很担心。
这个句子本身是事实，因此它就没有用到虚拟语气。
2、表情绪。
观点的形容词或名词也要用虚拟语气，如：necessary、important、impossible、natural、strange、surprising、funny、right、wrong、better、a pity、the shame、no wonder等。
句型：It is...that +主语从句，从句的谓语动词常用should+原形且should的省略要看情况而定。
eg: Do you think it is necessary that he (should)not be sent to Lingbao. (should表示“建议”的语气，可省。)
It is strange that such a person should be our friend.(should表示“竟然”的语气，不可省。)奇怪的是这样一个人会成为我们的朋友。
在从句中，我们也在表示紧急的形容词后面使用虚拟语气from。
这些语句类似于命令，但是它们没有人情味，因此更柔和。以下是一些表示紧急的形容词:可取的，最好的，关键的，可取的，必要的，迫切的，重要的，必要的，紧急的，至关重要的。
eg: It's vital that you make a decision right now. (Impersonal and softer)
Make a decision right now! (Strong)
I insist that you make a decision right now. ( strong)
注：这一点还没有准确的说法，希望善心人能把这点补充完全。
3、在even if, even though 所引导的让步状语从句中用may/might+动词原形，may/might可以省略，表示与现在相反的情况;从句用过去完成时，表示与过去相反的情况，类似的词有though/even though/whatever/however/so long as; 主句、从句的结构与if所引导的条件从句结构相同。
eg: Even if he were here himself, he should not know what to do.即使他亲自来也不知该怎么办。(事实：他没来)
Nobody could save him even though Hua Tuo should come here.即使华佗在世也救不了他。(事实：华佗不在世)
4、 在whatever，whichever，whenever，whoever, wherever，however, no matter wh-等引导的让步状语从句中，从句虚拟语气结构为：
指现 在或将来：may +do。
eg: We will finish it on time no matter what may happen.不管发生什么事，我们都要按时完成。
We will find him wherever he may be.无论他在哪里，我们都要找到他。
I will wait for him no matter how late he may come.不管他来的多么晚，我都会等他。
指过去：may +have done。
eg: You mustn't be proud whatever great progress you may have made.不管你取得了多么大的进步，你也不能骄傲。
We must respect him no matter what mistakes he may have made.不管他犯过什么错误，我们必须尊敬他。
5、一般would rather, had rather, would sooner等之后的宾语从句常表示与客观事实不相符的一种愿望，故使用虚拟语气。其虚拟语气的结构为：
过去 had + done
现 在 过去时(be 用were )
将来 过去时(be 用were) (would rather将来情况用一般过去时)
eg: I'd rather you had seen the film yesterday.我倒想你昨天看过了这场电影。
I'd rather you were here now.我倒想你现在在这儿。
We'd rather you went here tomorrow.我们倒想你明天去那儿
注：注意would rather，had rather，would sooner的细微差别，可以百度一下查查。
6、虚拟语气还可用在定语从句中，表示：“早该做某事了”时，定语从句中的谓语动词须用虚拟语气，其虚拟语气的结构为：It is (high / about) time that + 主语+ 动词的过去式/ should + do(should不可省略)(优先使用动词过去式)，即从句用虚拟过去式。
It is time that I went to pick up my daughter at school.我该去学校接我的女儿了。
It is high time you should go to work.你早该上班了。
7、 简单句中的虚拟语气
(1) 说话时，为了表示客气、谦虚、委婉而有礼貌，言语常使用虚拟语气。其虚拟语气的结构形式常为：would / could / might / should +do。
eg: Would you mind my shutting the door?你介意我把门关起来吗?
You should always learn this lesson by heart.你要把这个教训牢记于心。
I should agree with you.我本该同意你的观点。(委婉的不同意)
(2)表示“祝愿”时，常用may +主语+ do。
eg: May you have a good journey!祝你一路顺风。
May your youth last forever!祝你青春永驻。
(3)表示强烈的“愿望”、“祝愿”时，常用do。
eg: God bless us. 上帝保佑。
(4)习惯表达中常用的虚拟语气。
① 提出请求或邀请。
eg: Would you like to have a talk with us this evening?今天晚上来跟我们聊天好吗?
Could I use your bike now?我可以用一下你的单车吗?
② 陈述自己的观点或看法。
eg: I should be glad to meet you. 见到你我会很高兴。
I would try my best to help you. 我会尽力帮助你。
③提出劝告或建议。
eg: You'd better ask your father first.你最好先问一问你的父亲。
You should make a full investigation of it first.你应该先全面调查一番。
④ 提出问题。
eg: Do you think he could get here on time你认为他能按时来吗?
Do you expect he would tell us the truth?你期望他会告诉我们真相吗?
⑤表示对过去情况的责备时，常用虚拟语气。其虚拟语气的结构为：情态动词+ have done。
eg: You should have got here earlier. 你早该到这里了。
You should have returned it to him. 你早该把它还给他了。