This study explored students' grammatical, mechanical, and lexical errors in EFL writing. Also, it aimed to investigate the effect of instructor feedback throughout the semester on students' types and frequency of errors in two types of essays, including a process essay and an argumentative essay. This study was conducted on 24 EFL students studying in their first year of college in the applied linguistics department. To achieve the purpose of the study, the researcher used a descriptive qualitative study that dealt with document analysis. The author analyzed ten documents to understand how teacher feedback could develop students' levels in the target language and increase their abilities to properly use the grammatical, mechanical, and lexical rules in academic writing. Thus, the researcher used five written samples of students' process essays and five written samples of their argumentative essays to compare and find out about students' academic writing progress. The findings of this study revealed that the instructor's feedback positively impacted students' writing development and gradually helped them overcome the committed errors. There were significant differences in students' writing samples before and after the instructor's feedback. Therefore, EFL students' writing of the argumentative essays showed noticeable progress in students' language use and a reduction in the number of errors that students committed in their process essays.