This study employed 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 L2 learners in a self-financed tertiary institute in Hong Kong. Three intact classes of 47 students served as the experiment and control groups; the control group only received feedback on content and organization, whereas the two experiment 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 six pieces, among which four were analysed for the present research. The study found that students who received focused WCF significantly outperformed the other two groups, though the effects vary across error types. Meanwhile, no significant differences are found between the unfocused and control group. In-depth interviews explored how individual learners’ metalinguistic understanding and engagement affect the usefulness of WCF. The results reveal that learners who received focused feedback developed a deeper understanding of the linguistic nature of specific error types. Learners’ English proficiency and engagement strategies also played a role. Implications for pedagogical practice are discussed.