The goal of the paper is to identify the developmental pathways of iltan shaped by language contact with China. The study explores how the Sino-Korean term iltan evolved from a temporal noun into a discourse marker with distinctive functions. The study uses the perspectives of grammaticalization and pragmatics and employs historical and contemporary Korean corpora. Iltan originally meant ‘one morning’ and was used as a temporal noun in Late Middle Korean. However, its original meaning and noun status only persisted in Early Modern Korean until the 18th century. Its meaning ‘one morning’ was taken over by another Sino-Korean word, ilco , and purely Korean terms halwu achim and enu nal , which are all attested in the 15th century. As iltan shifted from a noun to an adverb, a connective, and a discourse marker, it acquired new meanings and functions associated with priority, short duration, and conditionality in Early Modern Korean. In Present-Day Korean, iltan is strategically used for multiple purposes, such as presenting topics and sub-topics, establishing common ground, filling pauses, indicating hesitation, and creating interruptions, thereby exerting its scope over a larger portion of the conversation. This transformation of iltan illustrates processes such as grammaticalization, discourse grammar, cooptation, (inter)subjectification, peripheral asymmetry, and source characteristics in comparison with the Chinese equivalent yīdàn . The findings provide insights into the Source Determination Hypothesis and its pragmatic implications.