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.”
当表示推测时候can/could have done 一般用在否定句和疑问句中,表示不相信或怀疑的态度.