"Is somebody there?"
I went to my friend's house today. I wanted to go to the bathroom, but the door was closed. I gave up once, but I thought maybe I should make sure if someone is there. So I aske my friend's mother. She said "I don't think so. Just knock the door." Well nobody was there.
I translated "there" to なかに which literally means "inside." I think this is more natural in Japanese. If you are talking to your close friend, you can use plain form, 中に、誰かいる？（なかに、だれかいる？）. Do not add か, when you are using a polite form for a question. It sounds rude or accusing. Look my book "Speak Like Native Speakers Japanese Verb Conjugation I" (46).
Autumn of Japan
6 years ago