This research stems from the importance of grammar comprehension, particularly in transforming active voice into passive voice, which often poses challenges for students. Errors in this process can affect the clarity of meaning and written communication. This research is a qualitative research that aims to identify students' error in changing active voice into passive voice, as well as to find out what type of tense students make the most error in changing active voice into passive voice. Data were collected through a writing test conducted in class X4, which was selected based on observations indicating that students in this class made more errors compared to other classes. The essay test consisted of 20 questions covering various tenses: simple present, simple past, present perfect, and simple future. The analysis revealed four categories of errors: (1) omission – the omission of essential elements in passive sentences, (2) addition – the inclusion of unnecessary words, (3) misformation – errors in verb form or structure, and (4) misordering – incorrect word order in passive sentence construction. The tense with the highest number of errors was the Simple Present (151 errors, 32.90%), followed by the Simple Past (149 errors, 32.46%), Present Perfect (89 errors, 19.39%), and Simple Future with the fewest errors (70 errors, 15.25%). In conclusion, these findings provide insights into students' error patterns, which can serve as a foundation for designing more effective grammar teaching strategies. Future research is recommended to further explore the causes of these errors and develop more suitable teaching methods to enhance students' understanding of passive sentence formation.