This paper explored Korean EFL learners’ use of English prepositions?especially those involving of?in written interactions with L1 speakers. Using learner texts from a Virtual Intercultural Exchange (VIE) project, this study tracked how the functional use of of evolved across three stages of interaction. Though the initial lens for functional development was a markedness perspective, results suggested that learners’ use of of reflected a more localized but dynamic interplay between strategic choices shaped by planning needs and discourse awareness. Analysis results showed that learners often fell back on routinized structures such as noun + of + noun patterns and fixed expressions that were typically grammatical but occasionally misaligned with pragmatic norms. As the exchange progressed, learners appeared to use of with increased contextual sensitivity in discourse-level functions, hinting a potential role of incidental learning in functional development beyond mere lexical use. These findings imply that ongoing, meaning-focused interaction in online spaces can help learners grasp abstract grammar functions. They also suggest mutual accommodation, as L1 speakers and L2 learners exhibited parallel trends in their use of prepositions over time. Altogether, these results underscor...