What could have happened?
1) could have done 表示对过去情况的推测. could表可能性常用于否定句和疑问句当中。 I couldn’t have been more than six years old when the accident happened.
2) could have done 也可表示本来能够做某事(事实上没做),表委婉的责备,常用于 肯定句。 He bought a ticket when he could have entered free.
2. 情态动词+ have done