疑问句：将must 提前就ok了
e.g I must go home.
Must I go home?
mustn't 是must 的否定词
而needn't 是must 的否定回答
e.g Must I go home?
No,you needn't.
must表示“有必要”时,附加问句通常要用needn't.
They must finish the work today,needn't they?
他们今天要完成这项工作,是吗?
而这句话是 我必须走了,是吗?所以用Mustn't了
应该是这样吧.,稍微有点小模糊