Chinese phrase
不是在〜 bú shì zài 〜
Meaning
Aren't you at ___?
When to use it
It asks whether someone is not at a place. It often checks an expected location and may show mild surprise.
Example dialogue
你 不是在 图书馆吗？ nǐ bú shì zài tú shū guǎn ma
Aren't you at the library?
我刚从图书馆回来。 wǒ gāng cóng tú shū guǎn huí lái
I just got back from the library.