If you said to your friend that she/he is a foolish, "너는 바보야" (you are so foolish) If you said that a person is very foolish to your friend, "he/she is very foolish" (걔 진짜 바보다) +++ "~야" is usually used when a person indicate familiar acquaintances (friend, family or the others) So if John and Marry are friends, you can say "John아, Marry야"