You're coming to the party tomorrow, aren't you?

aren't 是 are not 的缩写形式，are 是系动词 be 的人称变体，not 表示否定。you 是代词指代对方。aren't you 构成否定附加疑问标签，附加在陈述句末尾用于核实或确认说话者的预期。