could have done something 本可能做某事
例: I could have studied in Fudan University if I had done better in College Entrance Exam. 假如我高考考好一点的话,本来有可能在复旦大学念书的.
should have done something 本应该做某事 （该做的没做）
例："You should have handed in your homework this morning." the teacher said angrily. 老师生气地说：“你本应上午交作业的.”