1.表示与将来事实相反的条件从句中，谓语动词有三种形式：should+动词原形，were to+动词原形和动词过去式。其中值得注意的是should适用于所有人称，第二、三人称不能使用would。

If he would (应该为should) come tomorrow, I would show him around our school.

2. 在虚拟条件从句中，be的过去式各个人称及其单复数都用were

If I were you, I would not take part in the parade. 如果我是你，我就不会去参加游行。

3. If she had come yesterday, she would have had supper with us. 如果她昨天来， 就可以和我们一起吃晚饭了。

表示与过去事实相反的条件从句中，谓语动词用过去完成式，不能因为从句中有具体明确的过去时间就用过去式。主语谓语动词应用would/should/could/might等+have+过去分词。

4.如果虚拟条件从句的谓语动词含有were, had或should我们常可把if这个连词省略掉，将were, had, should放在从句主语的前面。

其他情况就不能省略if，倒装语序

If I knew Russian, I could have a talk with the Soviet tourist. 如果我懂俄语，我就会跟苏联旅游者交谈。

5. It is necessary that each middle school student should master a foreign language. 每个中学生掌握一门外语是必要的。

在It is necessary/strange/wonderful/important等+that从句中，主语从句的谓语动词是虚拟语气，常用should+动词原形表示，should可以省略，只剩下动词原形，但不能用其他情态动词代替should。类似的还有It is a pity/a shame/no wonder that…，It is suggested/desired/demanded that…以及动词“I drop caps”（insist, demand, request, order, propose, command, ask, prefer, suggest的首字母）

主语从句，宾语从句，表语从句，同位语从句中使用虚拟语气通常表示要求、建议、命令。

The teacher ordered that the test should be finished before nine.

His proposal is that the meeting should be postponed.

Your suggestion that she should be invited was rejected.

suggest表示“暗示”时，后面从句用陈述语气，表示“建议”时，后面从句用虚拟语气。

Are you suggesting that I'm too old for the job?
It's been suggested that children should take their exam early.

insist表示“强调”“坚持认为”时，后面从句用陈述语气，表示“坚持要求”时，后面从句用虚拟语气。

He insisted that the students talk in English with him.
He insisted that he was innocent.