The Lexical-Syntactic Interface Deficit: A Longitudinal Corpus Study on the Interplay of Grammatical Gender and Case Acquisition in L1 English Learners of German

The acquisition of the German case system, with its intricate reliance on grammatical gender, constitutes a paramount challenge for second language (L2) learners, particularly those whose first language (L1) is English. This study posits that the core difficulty lies not in the syntactic rules of case assignment per se, but in the unstable acquisition of lexical grammatical gender, creating a critical breakdown at the lexical-syntactic interface. Grounded in the Interface Hypothesis and Processability Theory, this research presents a longitudinal, mixed-methods investigation. We analyzed a 90,000-word learner corpus, tracking 80 L1 English learners from intermediate (B1) to advanced (C1) proficiency over 18 months, comparing 40 instructed learners with 40 naturalistic learners in a study-abroad context. Results from quantitative error analysis and qualitative case studies reveal that approximately 65% of all case-marking errors are directly attributable to prior incorrect gender assignment. This "cascading" error phenomenon is most prevalent in the Dative case and with feminine nouns, which learners often erroneously default to. While naturalistic learners demonstrated significantly better proceduralization of case-marked chunks (e.g., prepositional phrases), neither group showed significant improvement in the underlying, implicit knowledge of grammatical gender itself. The study concludes that pedagogical interventions must pivot decisively from teaching case paradigms in isolation to an integrated, input-rich, and lexically-focused approach that directly targets and strengthens the vulnerable gender-case interface.