could never have done 意思是“绝对不会做出那样的事”,和品德或承诺有关.属于虚拟语气的应用,未必是针对过去已经发生过的事.
例如:
I could never have done somthing like that to hurt your feeling.
could not do 有两种可能的用法,一是能力有所不及,一是有能力但是不愿意做.同样也没有特定时间的关联性.
例如:
I could not play piano even if I wanted to.
I could not do anything that may ruin my chance of getting that scholarship.