could (不能用 can) have done 表示“过去本能够做某事却未做.”注意：其否定形式 couldn't have done 没有虚拟语气的用法,couldn't have done 只能表推测,相当于 can't have done,意为：“过去不可能做了某事.”
举例：I think that he couldn’t/can't have gone abroad.I saw him just now.
我认为他不可能出国了.我刚才还看见他了.
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