This article analyses the experiences of 55 secondary pre-service teachers (PSTs) in a mandated professional development course that used Halliday's systemic functional linguistics (SFL) and related pedagogical practices to support PSTs' development of disciplinary linguistic knowledge for working with linguistically diverse students in the United States. Of specific interest is how the course influenced the manner in which PSTs responded to a student's attempt at writing a mathematical explanation. We begin with a brief description of Halliday's theory of language and learning with particular reference to the semiotic demands of mathematics. Next, we provide a description of the professional development course and the mixed methods approach to analysing changes in PSTs' pre- and post-course responses to a student writing sample about linear equations. Third, we discuss findings that suggest PSTs' emerging understanding of the linguistic and multimodal demands of mathematical explanations, supported them in providing more cogent and precise written feedback. We conclude by discussing the potential of SFL conceptual and pedagogical tools to support PSTs' professional development in the context of globalization.