This study aimed to investigate the effectiveness of a new instructional toolkit for improving English grammar skills, particularly verb tense structures, among Thai EFL students. The research objectives were (1) to evaluate the toolkit's effectiveness in enhancing students' English grammar skills in verb tense structures; (2) to examine the impact on Thai EFL students' learning achievement; (3) to assess knowledge retention after using the toolkit; (4) to analyze relationships among study levels, English proficiency levels, and learning achievement before and after toolkit use; (5) to explore students' perceptions of the toolkit's design, retention development, and implementation. The sample consisted of 200 students (100 primary and 100 secondary) from public schools in Bangkok's Thonburi district, using a multi-level group research design. Data collection instruments included the instructional toolkit, achievement tests, questionnaires, and interviews. Data were gathered before, during, and after the intervention, and analyzed using both quantitative (SPSS) and qualitative (content analysis) methods. The research revealed that 95 percent of students and teachers acknowledged the toolkit's efficiency and efficacy, with significant improvements noted post-intervention. The toolkit effectively enhanced knowledge retention in verb tense structures across different educational levels, showing particular resonance with higher-grade students and those at intermediate proficiency. Practical implications of the findings include (1) the need for school policies to mandate OBE-aligned toolkits with age-appropriate designs, and (2) the importance of incorporating self-paced, practical language application opportunities for Thai EFL students. This study contributes to discourses on instructional innovation in EFL contexts by underscoring tailored toolkits' potential to address specific linguistic challenges while aligning with educational reforms. Future research could explore adaptability across age groups, design variations, and other complex linguistic elements.