This research explores the morphological and phonological adaptation of the English agentive suffix '-er' into Korean, employing the framework of Optimality Theory (OT) by proposing a revised constraint hierarchy that accurately captures these processes. Through a comprehensive examination, two pivotal constraints, *NON-MORAIC-l and IDENT-IO (manner), were identified as central to addressing previous analytical shortcomings. The refined constraint hierarchy—comprising ALIGNSTEM- R, ALIGN-SUFFIX-L, *NON-MORAIC-l, SYLLCON, and IDENT-IO (manner, place)—offers a distinct explanation for the selection of optimal candidates in the adaptation process. This work not only deepens the understanding of phonological adaptation processes but also emphasizes the importance of adapting constraint hierarchies to accommodate empirical data, setting a foundation for future research in the field.