This paper aims to clarify the grammatical status of the so-called prefinal endings {었}, {겠}, {더} in Korean, which are traditionally described as temporal/aspectual/modal morphemes. From a general linguistics point of view, we examine how valid the concept ‘prefinal ending’ in Korean grammar is. The methodology used here is structuralism and psychomecanics of language. we pay attention to the fact that {었} and {겠} show the same distributional properties as the existential verb {있} does, when combining with the final endings and the connective endings. To the contrary, the distribution of {더} is restricted only to before the gerundive ending {ㄴ/ㄹ}. This observation leads us to the conclusion that {었} and {겠} can be analysed as auxiliary verb constructions {어ㅆ},{게ㅆ} in parallel with {-고있-}, {-지않}, {-어두-} etc. and {더} can be analysed as a prefinal ending. {었} and {겠] function as aspect element and modality element, respectively. {더} functions as a tense element.