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.为什么两句都用could，这几个词好象差不多，有什么区别吗？
热心网友
“can（could）+不定式完成式”的肯定式表示本来可以做而实际上未能做某事，疑问或否定形式表示对过去发生的行为怀疑或不肯定。You could have told me you were going to be late。你应当早告诉我你会晚到的。“may（might）+不定式完成式”表示对过去（将来）发生的行为的推测，表示可能干了某事。Who knows what will happen？You may even have married by then。谁知道会发生什么呢？到那时你甚至可能已经结婚了。“should+不定式完成式”表示本应该做而实际上没有做，其否定式表示某种行为不该发生但却发生了。I think he should have tried to get some more qualifications before applying for the jobs。我认为他在申请这份工作之前本应努力取得更多的资格证明。你自己用上面我说的方法套一下就知道为什么两句都用could了。