This study examined the part-of-speech classification of the Korean morpheme “ -ida”, which has been the subject of extensive debate in the history of Korean grammar, and compares it with the Japanese grammatical forms “ da”. In Korean, “ -ida” has traditionally been classified as a predicative particle, but some scholars have regarded it as a designated word, adjective, or even suffix. Such diverse classifications are attributed not only to the functional multiplicity of “ -ida” but also to the reception and transformation of Japanese grammatical categories during the formative period of modern Korean grammar. In standard Japanese grammar, “ da” have been consistently defined as auxiliary verbs of predication(斷定 助動詞), and there have also been discussions classifying it as an explanatory copula (説明存在詞) or predicative particle (叙述 助詞). This concept was subsequently adopted by Korean linguists, such as Choi Hyun-bae and Park Seung-bin, providing the impetus for establishing the part of speech of “ -ida”. Furthermore, “ da” was discussed in relation to its morphological similarity with Japanese na-adjectives(形容動詞). In Korean grammar, scholars such as Kang Bok-su proposed classifying “ -ida” as an adjective, though this view did not become mainstream. Through a comparison of “ -ida” and “ da”, this study demonstrated that the classification of “ -ida” was both a grammatical compromise in the formative period of modern Korean grammar and a historical product shaped by the introduction of Japanese grammatical categories. This study explored the historical background of Korean part-of-speech classification and critically examined the significance and limitations of grammatical categories.