"The dog returns after a year."
Translation:Hunden vender tilbage efter et år.
Technically, they both mean "return", but "returnerer" is mostly used when you are unsatisfied with something you bought at the store and want to get your money back. "Jeg returnerer varerne" = "I return the goods". "Vender tilbage" is only used in the process of coming back from another place, just like "kommer tilbage".