Purpose: This study adopted a mixed-method approach, including a classroom experiment and 24 in-depth interviews, to investigate the effects of two feedback techniques (coded focused and unfocused written corrective feedback) on ESL learners' writing in a self-financed tertiary institution in Hong Kong. Methodology: Three intact classes of 47 students served as the experimental and control groups; the control group only received feedback on content and organization, whereas the two experimental groups also received focused and unfocused linguistic feedback, respectively. The feedback intervention was conducted over an eight-week intensive summer course, focusing on three grammar errors (articles, singular/plural nouns and verb forms). Altogether, students wrote seven pieces, four of which were analysed for the present research. Results: The study found that students who received focused written corrective feedback (WCF) significantly outperformed the other two groups, though the effects varied across error types. Meanwhile, no significant differences were found between the unfocused and control groups. In-depth interviews explored how individual learners' metalinguistic understanding and engagement affect their intake of WCF. The results revealed that learners who received focused feedback developed a deeper understanding of the linguistic nature of specific error types. Learners with limited English proficiency were less likely to apply their linguistic knowledge to revise a task or write a new one. Conclusion: Because not all errors deserve equal attention, teachers and students should consider how feedback can be used more effectively, particularly in areas where comprehensive feedback is considered obligatory. When teaching students with limited language proficiency, it is recommended that, rather than providing a wide range of error corrections, teachers provide focused feedback complemented with carefully designed metalinguistic support. Copyright © 2022 National