But for the leadership of our Party,we(could/should) not have succeeded.As a good student,he (could/might) have gone to college last year,yet he got ill then and couldn't take the entrance exam.為什么兩句都用could，這幾個詞好象差不多，有什么區(qū)別嗎？

熱心網友

“can（could）+不定式完成式”的肯定式表示本來可以做而實際上未能做某事，疑問或否定形式表示對過去發(fā)生的行為懷疑或不肯定。You could have told me you were going to be late。你應當早告訴我你會晚到的。“may（might）+不定式完成式”表示對過去（將來）發(fā)生的行為的推測，表示可能干了某事。Who knows what will happen？You may even have married by then。誰知道會發(fā)生什么呢？到那時你甚至可能已經結婚了。“should+不定式完成式”表示本應該做而實際上沒有做，其否定式表示某種行為不該發(fā)生但卻發(fā)生了。I think he should have tried to get some more qualifications before applying for the jobs。我認為他在申請這份工作之前本應努力取得更多的資格證明。你自己用上面我說的方法套一下就知道為什么兩句都用could了。