Notice how 在 connects the place and the action.
在家 = at home, 在学校 = at school, 在书上 = on (the) book, 在床上 = on (the) bed.
If you want to sound mysterious, just say “我在这里” (I’m here) and leave.
Bonus:
Sometimes, you just want to say where something or someone is, and that’s fine:
(Gēge zài Běijīng xué Zhōngwén.)
Big brother studies Chinese in Beijing.
(Tā zài shǒujī shàng kàn diànyǐng.)
He watches a movie on his phone.
Ready? Let’s see if Lili, Jianguo, and the gang are where you think they are!
Lili is at the company.
Are you at home?
1. 浩然 学校打篮球。(Haoran plays basketball at school.)
2. 小彤 房间看书。(Xiaotong reads books in her room.)
3. 建国 家吃饭。(Jianguo eats at home.)
4. 莉莉 公司工作。(Lili works at the company.)
5. 志强 店里。(Zhiqiang is in the shop.)
6. 你 书上写字吗?(Are you writing on the book?)