One of the four fundamental abilities that students of English as a foreign language (EFL) need to develop is writing. Because it necessitates mastery of intricate syntax and structures distinct from the native language, writing is frequently seen as the most challenging of these abilities. Writing is scary because of this difficulty, and as a result, students frequently make grammatical errors in their compositions. In light of this problem, the current study sought to determine the most common errors in grammar made by students in high school as well as investigate the reasons behind them. Fifteen randomly chosen students served as the subjects. Students' written work in the form of cause-and-effect and analytical exposition texts served as the instruments, and a teacher interview was conducted to confirm the results. This study uses Keshavarz's error taxonomy and a qualitative descriptive methodology. The results revealed four types of errors: omission (55%), addition (21%), substitution (17%), and permutation (7%). The majority of errors were brought about by first language interference, in which students inadvertently employed their mother tongue's grammatical structures when writing in English. These results demonstrate that the primary cause of student grammatical errors is interlingual error. According to the study, in order to assist students in progressively improving their English writing skills, more targeted grammar training and integrated writing education are required.