This study, focusing on subject-verb agreement, investigates the correlation between Swedish upper-secondary students' English proficiency and their metalinguistic knowledge. The research adopts a methodological approach inspired by Karen Roehr's 2007 study, utilising both language proficiency and metalinguistic tests. Participants included Swedish students studying English 5, 6, and 7 in upper-secondary school. The study explores whether explicit productive and receptive knowledge correlates with students' metalinguistic understanding of subject-verb agreement. The study's results indicated a slight positive correlation between Swedish upper-secondary students' English proficiency and their metalinguistic knowledge, specifically regarding subject-verb agreement. However, none of the correlations found were statistically significant. This suggests that while there might be a link between English proficiency levels and metalinguistic understanding, the evidence from this study is not strong enough to confirm such a relationship definitively.