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情态动词 + 完成式不定式（have + 过去分词）用来表达对过去的判断："should have"（错过的义务）、"could have"（未实现的可能性）、"might have"（不确定的过去可能性）、"needn't have"（不必要的过去行为）。
You should have call me.
You should have called me. — 在 "should have" 之后使用过去分词，而不是原形。
You didn't need to bring a gift, but you did.
You needn't have brought a gift. — 对于本不必要却还是做了的行为，使用 "needn't have"。
should have studied
might have forgotten