虚拟语气应该是必修3第一、二单元的知识点.
常规下虚拟语气的考查点分两个：
一是名词性从句中should do结构,should可以省略.在名词性从句中,如果出现表示“命令,建议,请求、计划”的词,不管它用于什么名词性从句,都要求使用虚拟语气.如：
He suggested that we (should) finish the task on time.这是宾语从句,suggest是“建议”的意思.又如这词用于表语从句：
His suggestion was that we (should) finish the task on time.
在这种从句中,要注意一点就是：如果从句的主语和谓语动词之间是被动关系,那么就要用(should) be done结构,就和情态动词的被动形式一样.如：
He ordered that the task (should) be finished on time.
二是条件状语从句中的虚拟语气.从句和主句的谓语动词有严格规定,如下：
从句, 主句
和现在相反： did/ were would/could/ should/ might do
和过去相反： had done would/could/ should/ might have doe
和将来相反： did/ were to do/ should do would/could/ should/ might do
如：如果当时你没帮助我们,我们不可能完成这工程.
If you had not helped us then, we couldn't have completed the project.(与过去相反).
如果我是你的话,我会用另一种方法.
If i were you, I would use another way to do it.(与现在相反),这句中,要注意到我用的是were.尽管主语是单数,虚拟语气中的一般过去式,不能用was.
这些是基本规则,当然还有很多其他的.如：
I wish that.这个结构是个名词性从句中的宾语从句,但是它使用的虚拟语气不与我上面讲到的相同.这个句型中,宾语从句的时态要往过去推前一步.即：如果与现在相反,用一般过去时,如果和过去相反,用过去完成时,如果与将来相反,用过去将来时.
如：I wish that i were a bird.与现在相反
I wish that i hadn't wasted a lot of time before.与过去相反.
要全部讲具体,比较难,建议你系统看一下相关语法书会好些.