虚拟语气 if You could have done other career What would it have

情态动词和实义动词、助动词一样,也有虚拟语气,它可以表示与现在或过去事实相反的非真实条件句,可以用于if条件从句,也可以用于主句.当表示与现在事实相反时,只把情态动词变成过去式（即时态往前退一格）即可,表示与过去事实相反时,情态动词要变成过去式,后面接助动词have加上过去分词.如：
If I hadn't told you,you could have been killed.（情态动词的虚拟语气用于主句）
Joyce would have been astonished if she could have heard his voice.（情态动词的虚拟语气用于if从句）
I couldn't have been angry with him if I had tried.
你给出的句子,是属于与过去事实相反的非真实条件句,这里用了情态动词can的虚拟语气,所以连接词if后面不能用过去完成时,而是要把情态动词can变成过去时的could,再后接have加过去分词的结构.希望我能帮助你解疑释惑.