"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".
Once more: why can't we say here "repülnek az erdőbe" or in a previous sentence "egy tengerimalac maszik a vízbe"? Even after doing the course six times it is still very hard to understand when you put the verb in the end and when you don't. In such a small sentence how can you guess what you want to stress?