"The boyfriends and the girlfriends"
Translation:Τα αγόρια και τα κορίτσια
"Οι" is the definite article for plural masculine and feminine words in nominative, while "τα" is the definite article for plural neuter words in nominative and accusative. Both "αγόρι" (= boy, boyfriend) and "κορίτσι" (= girl, girlfriend) are neuter nouns, so you have to use "τα". :-)