Where: 在 and place words

在 zài says where someone or something is: the thing, 在, the place. Its negative is 不在 bú zài.

• 我在家。
  Wǒ zài jiā.
  I'm at home.
• 他不在办公室。
  Tā bú zài bàngōngshì.
  He isn't in the office.
• 你在哪儿？
  Nǐ zài nǎr?
  Where are you?

Place words

An ordinary noun becomes a place with a place word after it: 里 li "inside", 上 shàng "on", 下 xià "under", and the longer 里边, 上面, 下面, 前面 "in front", 后面 "behind", 旁边 "beside". Names of places, and nouns like 家 and 学校, are places already.

• 书在桌子上。
  Shū zài zhuōzi shàng.
  The book is on the table.
• 银行在学校旁边。
  Yínháng zài xuéxiào pángbiān.
  The bank is next to the school.

有 or 在

有 brings in something new at a place: "there is a cat in the room". 在 says where something already known is: "the cat is in the room".

• 房间里有一只猫。
  Fángjiān li yǒu yì zhī māo.
  There's a cat in the room.
• 猫在房间里。
  Māo zài fángjiān li.
  The cat is in the room.

Here and there

在 also goes before a verb: with a place, to say where something is done (Word order), and alone, to say it is going on (Ongoing actions).