The present study examines how L1-English learners acquire Korean subject honorification – a system that is socio-pragmatic in interpretation but syntactically constrained. Using a multi-method design (corpus analysis, politeness ratings, and self-paced reading), we find that learners show increasing sensitivity to politeness norms yet limited awareness of morphosyntactic constraints. In corpus analysis, learners used subject honorification almost exclusively alongside addressee honorification, indicating limited functional differentiation. In politeness ratings, learners consistently associated the subject honorific suffix with greater politeness, regardless of subject type, diverging from native speakers’ judgments. In self-paced reading, learners were sensitive to semantic anomalies (e.g., inanimate subjects) but not to morphosyntactic violations. Together, these findings suggest that learners interpret the subject honorific suffix as a general politeness marker, likely due to its low cue validity and frequent co-occurrence with pragmatically salient features. Our results highlight how cue reliability and competition shape L2 acquisition pathways under conditions of noisy linguistic representations.