"The birds fly into the forest."
Translation:A madarak az erdőbe repülnek.
It's a correct translation of the English sentence. There's only one mistake, probably just a typo: "Az erdőbe".
The difference between your sentence and the recommended one is that yours emphasises "a madarak": the birds fly into the forest, and not some other animals. Duo's recommended translation is neutral / slightly emphasises "az erdőbe".