Previous studies have indicated that students' writing skills benefit from contextualized L1 grammar teaching, in which language structures are observed and analyzed in authentic texts and as embedded into teaching of reading and writing. The contextualized approach is also promoted by the current Finnish curriculum for basic education. This study investigates the relationship between grammatical understanding and writing skills using statistical methods as well as a complementary qualitative analysis of student texts. The data are derived from a large cross-sectional assessment of L1 learning achievement in Finnish year 9 students (N = 6,044). Linear regression analyses indicate that grammatical understanding is a significant predictor of writing skills and correlates strongly with the syntactic, stylistic, genre-related, and orthographical quality of the students' argumentative texts. Weaker writers use less complex vocabulary and sometimes "lose control" of syntactic structures. Learning grammar is related to metalinguistic understanding which, in turn, helps writers to analyze and control their language use, and thus produce better texts.