在
zài
Expressing existence in a place with "zai"
在 as a main verb means "to be at" — no other verb required.
Subj. + 在 + Place
How it works
老师在上海 means the teacher is in Shanghai. 在 is the verb here, so 是 must not appear: 老师是在上海 is wrong.
Negate with 不 for a general state and 没 for a past one: 他不在家 (he's not home).
Known thing first, place second — the opposite arrangement to 有 sentences.
Examples
lǎo shī zài shàng hǎi
老师在上海。
The teacher is in Shanghai.
wǒ zài jiā
我在家。
I'm at home.
nǐ de shǒu jī zài nǎr?
你的手机在哪儿？
Where's your phone?