could后接动词的完成式,除表示对过去的推测外,还有以下用法(不用can)：
① 表示过去没有实现的可能性(即某事本来可以发生,却没发生)：
He could have told her,but he didn’t choose to.他本来可以告诉她的,但他却没有.
I could have lent you the money.Why didn’t you ask me?我本来可以借这笔钱给你的.你为什么不问我?
② 用来委婉地责备某人过去应该做某事而没有去做：
You could have been a little more considerate.你本可考虑得更周到些的.
You could ask me before you use my phone.你打电话之前可以问我一声.
You could have helped me why did you just sit and watch?你本可帮助我的,为什么只坐在一旁瞧着?
③ 表示“差点儿就要”：
I could have died laughing.我差点笑死了.
I was so angry I could have killed him.我是那样生气,差点把他杀了.
She felt miserable.She could have cried.她感到很痛苦,她差点哭了.