Changes in Text Difficulty in Korean 10th-Grade English Textbooks from the 5th to the 2022 Revised Curricula

This study examines changes in the text difficulty of 10th-grade English textbook passages across six South Korean curricula: the 5th, 6th, 7th, 2007 Revised, 2015 Revised, and 2022 Revised Curricula. In South Korea, textbooks are developed in accordance with national curriculum guidelines and their reading passages often function as the primary source of written English input. As such, they provide a valuable lens through which to examine policy-driven shifts in instructional materials. Using 18 textbooks, the researchers analyzed the main reading sections across four domains of text difficulty: word count, readability, lexical complexity (diversity and sophistication), and syntactic complexity (mean length of T-unit and clause). The results show that the 5th and 6th Curricula had similar text difficulty profiles, while the 7th featured shorter passages but higher lexical diversity. The 2007 Revised occupied a middle range across most measures. Text difficulty rose significantly in the 2015 revision and again in the 2022 revision, when readability increased, vocabulary became more varied and less frequent, and both clauses and T-units lengthened. The 2022 revision demonstrated the highest overall difficulty, aligned with its explicit standards emphasizing diverse vocabulary and complex structures. Overall, the findings highlight the influence of curriculum policy on linguistic demands and underscore the need for careful calibration of text difficulty in future reforms.