The difference between e.g. unter
and unten
is depending on what you are referring to and positions to each other. So the difference between unter
and unten
is the same as in under your foot
and down stairs
. So as a rule of the thumb I'd say, when ever you putting something in relation to something else, it should be really the preposition unter
, über
, vor
, hinter
etc.
If something is at a position not really related to something, than it's oben
, unten
, vorn
or hinten
. This also applies to positions inside e.g. a room: So you can say "Es befindet sich unten (im Keller)" means maybe something like "It's downstairs (inside the cellar)" whereas "Es ist hinter dem Keller" means "It's behind the cellar".
I have no clue how the grammatical thing is called ;)