We were all sure that it couldn't have been anything but a ghost!

couldn't 是情态动词 can 的否定形式，表示"不可能"或"不能"。 have 在这里是助动词，与过去分词 been 构成完成时态，表示"已经"或"曾经"。 couldn't have been 是一个虚拟语气结构，表示对过去情况的否定推测，意思是"不可能是"或"不可能是已经"。