Nobody knows where she lives,__________? You must do your homework by yourself,____________? You mustn't touch the machine,______________? 分别填什么？原理是什么？
Nobody knows where she lives,【do they】? 陈述句主语为everybody，nobody，anyone等时，疑问部分通常用they代替；陈述部分含有表示否定的词，疑问部分应该用肯定形式。 You must do your homework by yourself,【needn't you】? must表必须时，疑问部分用needn't
You mustn't touch the machine,【must you】? 有mustn't表示禁止时，疑问部分用must