Writing is a technical talent that calls for proficiency with both linguistic and organizational elements. The primary objective of this study is to examine the most common mistakes in argumentative essays written by ninth-grade students on an issue of "Advantages and Disadvantages of School Uniform." A total of ten essays were gathered in a controlled classroom setting and evaluated using an error analysis approach. The findings reveal that punctuation problems are most common (31.1%), followed closely by cohesion defects (25.9%) and distantly by grammar (21.8%) and lexical (21.2%) errors. The results indicate that students' most struggle comes with text arrangement and writing mechanics in comparison to what they do with grammar proficiency itself. The study highlights the critical importance of targeted pedagogical interventions to improve students' overall writing competence. Introduction Writing is typically recognized as one of the most demanding skills in English as a Foreign Language (EFL) teaching. It requires learners to incorporate grammatical knowledge, vocabulary, punctuation, and logical sequencing into a coherent written work. Writing an argumentative essay faces an additional challenges for secondary school students, particularly those in the ninth grade, as it involves opinion expression, idea comparison, and maintenance of balanced structure. The topic "Advantages and Disadvantages of School Uniform" is commonly used in the classroom practice since it facilitates student's expression of both positive and negative perspectives. However, despite their familiarity with the topic, many students struggle to generate accurate and well-organized essays. Error analysis offers valuable insights into learners' language development and identifies areas that require instructional support. Therefore, this study aims to address the following research question through investigating the most frequent mistakes made by students in their writing: What are the primary linguistic obstacles that ninth-grade EFL students confront while composing an argumentative essays? Literature Review Error analysis has historically been acknowledged as a crucial approach in second language acquisition. According to Corder (1967), errors are natural and essential component of learning process since they reflect learners' developing competence. Similarly, Brown (2007) highlights that mistakes provide crucial details about how language norms are adopted by students. Hyland (2003) emphasizes that mastery of discourse structure genre rules is just as important to good writing as grammatical correctness. In argumentative writing, students must use cohesive to guide the reader and properly arrange concepts to guide the reader. Ellis (1997) states that learners often undergo struggles in multiple areas simultaneously, including grammar, vocabulary and coherence. Prior studies have demonstrated a frequent trouble with cohesion and punctuation, which significantly affect the clarity and readability of the writing. Methodology 3.1 Reaserch Design This investigation implements both qualitative and quantitative error analysis techniques to examine students' writing. 3.2Participants Ten ninth-grade students learning English as a foreign language participated. Their competence level ranged from lower-intermediate to intermediate. To guarantee anonymity, students were labeled S1-S10. 3.3 Task Design Students completed a precise argumentative writing in a classroom setting. Topic: Advantages and Disadvantages of School Uniform Word limit: 120–180 words Time limit: 20–25 minutes Conditions: No dictionaries or translation tools were allowed This method verified for the accurate data representation of learners' autonomous writing. 3.4 Data Collection and Analysis A total of ten essays were collected and manually analyzed. Errors are divided into four categories. Grammar: subject-verb agreement, article usage, sentence structure Lexis: vocabulary choice, repetition, incorrect word forms Cohesion: linking devices, logical flow, organization Punctuation: commas, full stops, capitalization Calculations were made for combination of frequency counts and percentage distributions. Results 4.1 Quantitative Findings The analysis identified a total of 193 errors across the students' essays. Punctuation: 60 errors (31.1%) Cohesion: 50 errors (25.9%) Grammar: 42 errors (21.8%) Lexical: 41 errors (21.2%) 4.2 Summary of Findings The findings indicate that punctuation is the most troublesome area, followed by cohesion, while grammar and lexical errors occur at similar but slightly lower rates. Discussion The results obtained suggest that punctuation errors represent the most significant struggle for students. Frequent errors in capitalization, commas, and full stops indicate that learners lack a clear understanding of basic writing strategies. This weakens overall readability and results in ambiguous sentence boundaries. The second major issue lies on cohesion, stresses students' challenges in organizing ideas logically. Many essays fail in clear paragraph structure and appropriate linking words. Instead of presenting a string of paragraphs as well as well-developed arguments, a series of disconnected sentences are produced. This affirms Hyland's (2003) view that writing competence involves more than grammar; it requires control over discourse structure. Grammar errors, accounting for 21.8%, remain a notable issue. Subject-verb agreement, incorrect sentence construction, and misuse of articles are common issues. These errors are characteristics of typical developmental patterns that Brown (2007) described. Similarly, lexical errors point to a narrow vocabulary. According to Ellis (1997), who highlights the significance of vocabulary in efficient communication, students often rely on simple and repetitive words, which limits their ability to express complex ideas. Overall, the findings imply that mechanics (punctuation) and structure (cohesion) rather than grammar alone are the main causes of students' writing challenges. Conclusion This study investigated the writing errors of secondary school EFL students and identified four major categories of difficulties. The findings reveal that punctuation (31.1%) and cohesion (25.9%) are the most problematic areas, followed by grammar (21.8%) and lexical errors (21.2%). These outcomes imply that a comprehensive approach that goes beyond teaching grammar is needed to improve students' writing. Greater emphasis should be placed on teaching punctuation rules, developing cohesive writing skills and expanding vocabulary. Consequently, this pattern may have broader impact on students' academic performance since clarity, coherence, and task achievement directly demand for strong punctuation and cohesion. To address these issues, it is recommended that teachers: 1) provide explicit instruction on punctuation and sentence boundaries; organize activities that develop cohesion and logical organization; encourage the use of diverse vocabulary offer regular, detailed feedback on students' writing These techniques can significantly enhance students' capacity to produce precise, coherent and well-structured argumentative essays.