The purpose of this study was to identify the most common grammatical issues encountered by students in different categories such as word choice (vocabulary), singular/ plural, verb tense, articles, capitalization, preposition, and the classification of errors such as omission, addition, misordering and misinformation. Moreover, this study looked at the grammatical errors in English essays written by Grades 8-10 students of Modified Work and Study Program (MWSP) in Kapalong East District, Schools Division of Davao del Norte using a corpus-based technique to discover, categorize, and analyze recurring patterns of error. A corpus of 51 essays was collected, classified, identified, and evaluated using an error-analysis approach. The findings found that verb tense and misinformation were the most common committed errors. These were followed by improper use of vocabulary, preposition, capitalization, and articles as well as incorrect order, excessive use of words in a sentence. Accordingly, these errors were caused by a lack of exposure to formal English usage, limited interaction of ESL learners and teachers, decrease of opportunity and a poor knowledge of basic grammatical rules. This study emphasized the need for specialized teaching strategies and intervention programs that address the need and level of ESL learners especially in the Modified Work and Study Program (MWSP). By addressing these challenges, the study hoped to improve students' written communication abilities and overall academic achievement. Hence, the results contributed to the expanding corpus of research on error analysis in English as a Second Language acquisition.