I have just arrived by train这里为什么要用现在完成时呢

I have just arrived by train这里为什么要用现在完成时呢 i have just arrived by train.为什么有have.它起什么作用 表状态情绪感觉精神活动的动词,不用现在完成时对吗～I have just arrived by train 这句话对吗 ‘I’ve just arrived by train中的 ‘I have just arrived by train,’she said.变成间接引语 he will have arrived by this time tomorrow.为什么要用现在完成时? You have just missed the train.这句为什么要用现在完成时 I've arrived by train为什么不是I arrived by train? I have just arrived in China为什么要加 have i have arrived by train中的arrive 为什么用完成?而不用be there? i have just arrived in china为什么加个have i arrived there just as it was beginning to snow.谁能告诉我这里为什么用