This study examined the interplay among second language form type (easy vs. difficult), the individual difference factors captured by the new Second Language Motivational Self-system (L2MSS), and the type of L2 knowledge (explicit vs. implicit), focusing on how newly bifurcated self-guides relate to knowledge of two target forms, using a quasi-experimental design. Participants received explicit form-focused instruction (deductive) on easy and difficult L2 forms, followed by explicit and implicit knowledge tests for the two forms and completion of the new L2MSS questionnaire. The sample comprised 259 Iranian ESP university students (134 males, 125 females), aged 18–25, with an average pre-intermediate English level. Participants were recruited via convenience sampling. Two target forms (easy and difficult) were tested for explicit/implicit knowledge. The Roehr and Ganem-Gutierrez’s (2009) taxonomy of form difficulty and Papi et al.’s (2019) new four-factor model of L2MSS were utilized to examine the interrelationships between type of L2 forms, type of Iranian EFL learners’ motivation, and type of L2 knowledge. SEM analyses were conducted to examine direct, indirect, and total effects among L2MSS sub-dimensions and explicit/implicit knowledge across form difficulties The findings illustrated that; ideal L2 selfown and ought-to L2 selfown were significant positive predictors of explicit and implicit knowledge, and the explicit knowledge was a significant positive predictor of implicit knowledge of both easy and difficult L2 forms. The outcomes underscore the substantial role of bifurcated L2MSS self-guides in shaping explicit and implicit knowledge, with explicit knowledge facilitating implicit knowledge. These results have implications for designing form-focused instruction.