Internationally, English curriculum increasingly emphasises the explicit teaching of knowledge about language. This paper reports on a small pilot study of an ongoing project, investigating the relationship between explicit knowledge and its uptake by students. Drawing on the underpinnings of Vygotskian sociocultural theory and Hallidayan systemic functional linguistics, this paper presents an interdisciplinary approach to the analysis of development in students’ metalinguistic understanding. The approach recognizes the centrality of the expert’s mediation via classroom interaction involving data drawn from lesson observations, interviews and writing conversations with students. The findings demonstrate some of the complexity involved in learning a grammatical concept; that is, in recognizing, acquiring and applying metalinguistic knowledge to the task of becoming literate. The paper argues that the purpose of teaching metalinguistic knowledge should be to support learners to become confident and successful users of language. Our findings also make some important suggestions for pedagogic practice with respect to teaching knowledge about language to young learners.