could have done 是否既有表对过去以完成动作的推测 也有表过去可以做.却未作 怎样在做题中区分是那

1.根据上下文的意思判断.
比如：We could have visited the palace,but it was raining hard.
我们本可以去参观那个宫殿的,但是当时下大雨（所以没去）
这里的could have done 表过去可以做.却未做
---- The five of us were trapped in the dark cave for two days.
---- That couldn't have been a pleasant experience.
---- 我们五个人被困在那个黑乎乎的山洞里整整两天.
---- 那种经历肯定不好受.
这里的could have done 表对过去发生事情的猜测.
2.could have done 表对过去发生事情猜测的时候,只能用在否定句中,即couldn't have done.如果要表对过去发生事情肯定的猜测,用must/may/might have done.
比如,You must have been very excited at the news.
你当时听到这个消息一定很兴奋吧.
You couldn't have seen him at the party last night,because he was ill in bed.你不可能在昨晚的聚会上看到他的,因为当时他正病着.

一般都是表示前种，后者在口语中有出现过，但是不会在书面上用太多，表示本应做而没做一般都用should do/should have done