This study investigated a Korean child’s interim knowledge and use of English verb inflections in the context where acquisition is based mostly on written input with limited opportunities for oral production and interaction. The emergence and development of the child’s interlanguage were closely examined by analyzing the occurrences and frequency changes of (non-)finite verb forms and their appropriateness in context. Data were collected for about a year from the child’s uninstructed spontaneous compositions and analyzed across four time periods. Error classification systems were developed to classify accurate, inaccurate, and formally accurate but functionally inappropriate forms. Overall, the findings indicate that errors in the child’s developing language are restricted to only a few types which are also observed in English L1 acquisition, and that the child’s L1 does not mediate her acquisition of L2 morphosyntactic features to a significant degree. These seem to imply that child acquisition of L2 grammaticality is driven by sufficient input even when it lacks the qualities of social scaffolding and interactive communication.