Translation:The young Russian girl is walking in the department store.
in the original, the adverb "in the department store" is referring to the act of walking, and in yours, it is referring to the girl.
my translations to your version:
- Az áruházban lévő fiatal orosz lány sétál. (lévő = being)
- A fiatal orosz lány, aki az áruházban van, sétál.
That makes sense! Does the location for -ban always apply to the action like that? For instance:
- A férfi a házban főz - The man is cooking in the house
- A katonák harcol a folyóban - The soldiers fight in the river
(Also incidentally, if there's a preposition like mellett, is it just for example "a folyóban mellett" or does something else change?)
- A férfi a házban főz
- A katonák a folyóban harcolnak
Yes, these examples are correct, and with -ban you make these actions take place inside of something.
You could use the postposition mellett on its own, like "a folyó mellett" = "beside the river", but sometimes their use is overlapping with (rag type) suffixes. eg. "a folyónál" = "at the river".