이거든 is not a word. e.g. Imagine when you walk on the street with your friend. Then you see a character performer. Your friend tell you " You know what? This is not a human." and you can say "이거 사람이거든?" 이거든은 그냥 문장 끝에 붙는 말입니다. ~거든이라는 말은 자주 쓰여요. 이거든 뿐 아니라 예를 들어 "이거 내꺼거든?"으로도 쓰이구요. I'm not a Korean teacher, just a native Korean spaker and i can't tell you this expertly. Therefore, this just what i know. :)