This is the 8475th most frequent Korean word.
ㄴ
"ㄴ" is a consonant in the Korean alphabet (Hangul), representing the "n" sound.
Here, 'ㄴ' is part of '나는', indicating the subjective particle of the pronoun '나' ('I').
나는 학생입니다.
I am a student.
The 'ㄴ' is part of '좋은', showing a descriptive adnominal form of the adjective '좋다' ('good').
좋은 아침입니다.
It is a good morning.
The 'ㄴ' again forms the descriptive adnominal form '좋은,' used here with negation by '안', creating the meaning 'not good.'