Nobody could ever replace you为什么用could不用can

Nobody could ever replace you为什么用could不用can Nobody could ever replace you Nobody could ever replace Nobody could ever replace you.啥意思? 别人对我说 Nobody could ever replace you. Nobody can replace you? You,nobody can replace. Nobody can replace you could you come with me,please?这里为什么用could 而不用can 呢? g nobody can replace you Could you take the photo here tomorrow?为什么不用can? I guess I could come over.为什么用could不用can呢