This is the 119th most frequent Korean word.
아니다
"To not be" or "is not."
Here, '아니다' is used to negate the noun '책' (book), indicating what something is not.
이것은 책이 아니다.
This is not a book.
'아니다' is used to explain the speaker's identity, negating the description '학생' (student).
나는 학생이 아니야.
I am not a student.
In this case, '아니다' negates a relational statement, emphasizing what the subject's attribute or relationship is not.