What does it mean when you end a statement with なんだ? For example, 「犯人は君なんだ。」

Answer 1

You

 From what I learn that when you end a statement with なんだ is to emphasize this sentence. 犯人は君なんだ actually is the transform from 犯人は君なのです。When you add のですbehind a sentence usually is for emphasizing and you need add な to connect 犯人は君 and のです。and when you use なのです together then なのですwill transform to んだ。