["European Journal of Education, Volume 61, Issue 3, September 2026. ", "\nABSTRACT\nSelf‐concept and self‐efficacy have received much attention from second/foreign language (L2) researchers. However, very few studies have empirically deciphered their similarities and distinctions. Even fewer have investigated how their contributions to L2 achievement vary across L2 proficiency levels. The current study examined the commonality and specificity of self‐concept and self‐efficacy and their relative contributions to English achievement and their fluid effects on English achievement across different proficiency groups based on the Island Ridge Curve theory. A sample of 369 Chinese undergraduate students completed two questionnaires of self‐concept and self‐efficacy, and their English achievement was represented using College English Test Band Six scores. Bifactor structural equation modeling (SEM) revealed that the combined scale shared one commonality factor and two specific factors. Multi‐group bifactor SEM found that the general factor and specific self‐concept factor were positively related to English achievement and their respective effects fluctuated across English proficiency groups.\n"]