"The cat eats the boy's birds."
Translation:Katten spiser drengens fugle.
Drengen = the boy Drenge = boys
Dreng is the noun stem. En dreng is "a boy". For the definite article, add "en" to the end to get "drengen". The plural just adds "e" to both the indefinite and definite forms - so "drenge" (boys) and "drengene" (THE boys).
So the translation is entirely correct here. The cat (singular definite - katten) eat's the boy's (singular definite + genitive - drengens) birds (indefinite plural - fugle).
It's a fairly simple system in Danish (and likewise in its close relatives, Swedish and Norwegian), but it can still take a bit of getting used to, especially if you're familiar with languages like German and Dutch where the "-en" suffix often DOES indicate a plural.