"Beautiful, white birds fly across the sea."
Translation:Szép, fehér madarak repülnek át a tengeren.
In this sentence "Szép, fehér madarak repülnek át a tengeren." we split the word átrepülnek. In the previous example "A fekete hajók átúsznak a tengeren" we didn't. What is the difference?
Emphasis. When the verb is not split, it is emphasized. When it is split, the word in front of it is emphasized.
For this sentence, imagine the question: "What flies across the sea?" - Asking about the subject in front of the verb.
For the "fekete hajók", imagine this question: "What do the black ships do?" - Asking about the verb, the activity that the ships perform.