im not korean so don't take my word on it but i think when the last consonant is a ㄹ it's dropped and then you just add the -ㅂ니다 to the 나 to get 납니다 like you'd do if there was no ㄹ and it ended with a vowel. you can probably find videos on this on YouTube though
I got it to work (and another glitchy exercise) to register when I pronounced the ㄴ like a very strong English /d/ and not at all like an /n/. shrug It's still confusing me a bit because that's not how the person says it!
i think it's because since there's a 가, that means you're not talking about butterflies in general, and since there's no 들 you're not talking about multiple butterflies so therefore it's butterfly, singular