Imagine yourself going to a lecture and you get there late. The place is full, practically everyone else is sat down and you can't see anywhere to seat yourself. You go up to someone who looks vaguely like they know what's happening and ask je kuna siti?
They scan the area, then point to a row of chairs about a third of the way from the back. Pana siti. You thank them, make your way to that precise location, and sure enough there is an empty seat.
You ask with kuna because you do not know the location of the seat, if there is one, just that it should be in this vicinity. It is indefinite. The response here is pana because you have been told where it is. The location is definite.
This isn't a perfect demonstration of the difference, but I hope it gives you an idea. We'll assume for now that the lecture was outside, so we don't have to confuse the issue with mna (used for inside/enclosed locations).
Evening inside a room your example holds. Mna is used inside closets/pockets/bag (small things) and in specific circumstances for countries, but mostly inside the water; inside a bottle etc.