According to my book, Korean Grammar in Use, "구나" is the plain informal version of "군요." You use these endings to express a sudden realization. Examples from the book: "감기에 걸렸군요." = "I see you've caught a cold." "정말 비가 오는군요." = "Wow, it's really raining." "응, 그래서 늦었구나." = "OK, so (I see) that's why you're late."

안녕하세요. 저도 한국어를 4년동안 배웠어요. maybe we can study korean together and help each other. I like to make diffident friends.