This study provides two exemplars of how grammatical analysis of language use in elementary science classroom discourse enhances the explication of students' epistemic process in the language use in that discourse. Methodologically, both practical epistemology analysis (PEA) and discourse register analysis (DRA) from systemic functional linguistics were employed as a combinatory approach. The former was used to construe epistemic process of students' language use and the latter was to articulate grammatical features of the language use. The exemplary analysis was administered for the discourse data collected at two elementary science classes in South Korea. The exemplars showed that ideational metafunction and its semantic relation analyses in DRA facilitated identifying "gaps" and "relations" in PEA, and interpersonal and textual metafunction analyses in DRA supported construing the flow of gap-filling or lingering process in PEA. Analysing students' language use by PEA combined with DRA can help science teachers find the points where the language use in school science gets epistemological productivity, while it is different from the language game in science disciplines.