When were they there?为什么不说“When were they at there?”

When were they there?为什么不说“When were they at there?”
句子是新概念英语的,字典上说there adv.在那里 n.那里
本句中there在句子是名词吗?
为什么不用When were they at there?
When were they there?直接翻译的话不就成了“他们什么时候那里?”了吗