This study discusses the part of speech of ‘있다,’ the Korean word for ‘to existentially be,’ and identifies it as an adjective. To this end, it reviewed previous topics of discussion, and examined the uses of the word. Then, this study approached the unique usage patterns of the word from a historical perspective. The discussed topics largely fall into two categories. Firstly, this essay argues that the word, ‘있다,’ in its conjugated usage at a sentence’s end, is much closer to an adjective than a verb. Moreover, in previous discussions, it was argued that, in order for the word to be used like a verb, the subject must be an animate noun. However, this study takes another step forward in purporting that, even if the subject is an animate noun, the word ‘있다’ maintains its ambiguity as a possible adjective and possible verb. Based on this, it is argued that the cases in which the word appears to be verb-like is very limited. Secondly, this essay argues that the use of the word ‘있다’ in its adnominal form seems like a verb in appearance, but is somewhat distinct from a verb. The use of locative case markers‘-에서’ and ‘-에’ are presented as evidence. Additionally, in order to elaborate further on this argument, it is explained how it is possible for the word ‘있다’ to be combined with the pre-final ending‘-느-.’ To this end, combinations with ‘잇-/이시-’ and ‘-□-’ found in Korean language historical materials were discussed. After presenting the high frequency of these combinations and, on those grounds, drawing attention to the semantic qualities, or constancy, that is retained by existing predicates, it was derived that the reasons for why there is a high frequency of ‘잇-/이시-’ and ‘-□-’ combinations lie in semantic drivers.