아니다
anida · TOPIK 1
to not be
아니다 = 아니 (no) + 다: the word for no, turned into a verb
아니 is the everyday Korean no, and 다 is the dictionary verb ending. So 아니다 is literally no-ing: to not be. It is the direct opposite of 이다 (to be), and its polite form 아니에요 means it is not.