Positions and locations with 在

Describing position or location almost always takes two elements. First, let’s look at the verb 在. It means “to be located at a place.” If no such place is specified, then it just means “is there, exists.”

他在吗? 他不在.
Is he there? He’s not there.

Once we get more specific about a position in space, we almost always have to include a post-position after the location.

The cup is (located) on the table.

The school is behind (lit. on the behind-side of) the hospital.