The study explored the impact of proficiency, frequency, L1-L2 congruency, and semantic transparency on the knowledge of multi-word units (MWUs) among Iranian L2 learners (N = 256). A gap-filling test was used to assess learners' productive knowledge, employing a high-frequency MWU list created with the lemmatised concgramming method. The list, which included 11,212 MWUs across four frequency levels, was ranked by L1-L2 congruency and semantic transparency. As revealed by regression models with bootstrapping, the results showed a significant positive correlation between MWU knowledge and proficiency (IELTS scores). It was observed that knowledge of MWUs decreased with lower frequency, and participants scored higher on congruent and transparent items. The study also found that frequency and semantic transparency influenced IELTS scores, with a significant interaction between congruency and incongruent items. These findings underscore the crucial role of L1-L2 congruency in learning MWUs and provide hope for educators, as they offer insights to enhance learning by prioritising L1-L2 congruency in English MWU resources. This study's findings can potentially lead to improved language teaching methodologies and curriculum design, offering a pathway to optimise learning outcomes.