subjunctive mood 和unreal condition 有什么不同

I'm sorry I can't sing.(Subjunctive mood)
A I could sing
B I want to sing
C I wish I could sing
D I wished singing
E I can to sing

专四英语语法(subjunctive mood)
if there were no subjunctive mood,english ___ be much easier to learn.(b)
a) could have been b) would be c) will be d) would have been
had judy been more careful on the maths exam,she ___ much better results now.(b)
a) would be getting b) could have got c) must get d) would get
这题的解释是不是,
虚拟的从句是一般过去式,主句用would、could、 should、might+原型（将来时）
从句是过去完成时,主句用would、could、 should、might+完成式啊?
而且在网上找到别的说什么这两个不同是因为动词不一样,get是一个点,所以用could have got;
be是状态,所以用would be
求***,求详解