There are two linking verbs in Korean - 이다 and 아니다.
이다 is equivalent to the English verb "to be", while 아니다 is the opposite and can be translated as "not to be".
These linking verbs have almost the same grammatical categories as other verbs, but they have their own specific forms.
A subject that is placed before the linking verb 이다 has no case suffixes, but a subject that is placed before the linking verb 아니다 is used in nominative case.
Example:
이것은 책입니다. "This is a book."
이것은 사전이 아닙니다. "This is not a dictionary."
See Also: