Patterns and Progress in Idiom Use: A Longitudinal Analysis of Thai EFL Learners'Writing

This longitudinal study examines the developmental patterns of English idiomatic expression use in the written productions of Thai EFL learners over three academic years (2022–2024). The objectives were to determine the extent of idiom use, identify frequency patterns, and track changes in usage over time. Analyzing 464 essays from 58 undergraduate students, the study employed content analysis to classify idioms into five types: opaque, transparent, semi- transparent, phrasal verbs, and proverbs. A total of 771 idiomatic instances were identified, with 525 unique expressions remaining after eliminating repetitions. The findings revealed a quantitative increase in idiom use over time, with transparent idioms and phrasal verbs being the most prevalent, while proverbs were the least used. However, this growth was not accompanied by qualitative improvements in accuracy or sophistication. Students consistently favored simpler, more transparent idioms (e.g., "break the ice") over opaque or culturally nuanced ones (e.g., "spill the beans"), reflecting cognitive and cultural challenges. The study highlights the need for explicit instruction to enhance idiomatic competence, as prolonged exposure alone proved insufficient for mastering figurative language. These results align with prior research on EFL learners'preference for high-frequency, analyzable idioms and underscore the importance of targeted pedagogical strategies to foster deeper idiomatic proficiency. The study contributes to the understanding of idiom acquisition in Thai EFL contexts and offers practical implications for language instruction.