아니다 is the Korean verb meaning "to not be" (yes, it is a separate verb rather than having the word "not" placed in front of 이다). It is used to link a subject with a predicate (also called a copula in Linguistics) but with negation built in to the meaning. Based on setting and speech register, you will it appear in numerous forms but the three most common are listed below:
아니다(dictionary form, very casual)
아니에요 (generically formal, polite)
아닙니다 (humble form)
You will use 아니에요 for the vast majority of your communication in Korean.
Survival Korean
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Survival Korean