This study aims to identify and analyze the common errors found in the English letters written by first-year English major students at Hanoi University of Natural Resources and Environment. The analysis focuses on errors at both the sentence and word levels, such as punctuation, subject-verb agreement, articles, and spelling. The data were collected from 24 written letters, and the findings reveal that most errors stem from interlingual interference, intralingual interference, limited knowledge of English grammar and vocabulary, and carelessness. The study provides pedagogical implications for improving students’ writing skills, emphasizing the need to address both linguistic knowledge and writing habits.This study aims to identify and analyze the common errors found in the English letters written by first-year English major students at Hanoi University of Natural Resources and Environment. The analysis focuses on errors at both the sentence and word levels, such as punctuation, subject-verb agreement, articles, and spelling. The data were collected from 24 written letters, and the findings reveal that most errors stem from interlingual interference, intralingual interference, limited knowledge of English grammar and vocabulary, and carelessness. The study provides pedagogical implications for improving students’ writing skills, emphasizing the need to address both linguistic knowledge and writing habits.