第一步：掌握对现在，对过去，对将来的虚拟的固定时态
对现在的虚拟：
If he were free, he would help us. 要是他有空的话，它会帮助我们的。
注意，虚拟中的is,都用were
注意，主句用would +动词原型，从句用
过去式对过去的虚拟
If I had seen the film, I would have told you about it. 我如果看过这场电影，我会把电影内容告诉你了
注意时态：主句用would +
现在完成时，从句用
过去完成时对将来的虚拟
If he were to come here, he would tell us about it.
如果他要来的话，他会通知我们一声。
或，if he came here...
if he should come here....
主句用would +动词原型，从句分三种：过去式，were to,should
第二步：掌握错综虚拟，即
主从句的动作（看时态提示词），有错综
If he were free today, we would have sent him to Beijing.
如果他今天有空的话，我们会已经派他去北京了。
本来主句用了would +现在完成时，说明是对过去的虚拟，从句本来应该用过去完成时，但是，由于从句中有个时态提示词，today,则套用对现在的虚拟的公式，用过去式
If he had been working hard, he would be working in the office now.
要是他工作一直努力的话，他现在已进了办公室了。
从句用了过去完成时，说明死对过去的虚拟，本来主句要用would +现在完成时，可是，主句里有个时态提示词，now,则主句套用现在的虚拟，用would +动词原型
这样根据主句，从句不同的时态，针对不同时间的虚拟，叫错综虚拟
第三步，含蓄
虚拟语气，就是把if 省略，用倒装
Were she here, she would agree with us. 如果她在这儿的话，她会同意我们的。
=if she were here,she would agree with us
第四步，虚拟语气再
名词性从句中的应用
对于表建议，命令，要求的词，虚拟比较固定，一律用should +动词原型，但是should 可以省略
advise, ask, demand, desire, decide, insist(坚决要求) , order,
propose, request, suggest(建议) 表示请求、要求、命令或建议等意义的动词所接的
宾语从句一般用虚拟语气，其虚拟语气的结构为：(should) + 原形动词。如：
The teacher advised that we （should） make good use of every minute here.
老师劝我们要好好地利用在这儿的每一分钟。来自：求助得到的回答