COMPASS is an e-learning system that can visualize grammar errors during sentence production in German as a first or second language. Via drag-and-drop dialogues, it allows users to freely select word forms from a lexicon and to combine them into phrases and sentences. The system's core component is a natural-language generator that, for every new word the user wishes to attach to the current string (as an extension of this string or as a replacement of a substring), checks whether this tentative attachment is grammatically well-formed or not. On this basis, the system can compute and display online the grammatical structure of input strings in the form of syntactic trees, and identify and diagnose input errors. In the following, we focus on the crucial question of how to present the feedback to the learner. We propose tutored visualizations with animations of pedagogical agents. We briefly report the results of a preliminary user evaluation study in which the participants judged the well-formedness of prefabricated input sentences. The data, collected by means of eye-tracking and a questionnaire, show that L1 learners who are exercising an unfamiliar and error-prone grammatical structure, pay due attention to, and can profit from, this type of visualized error feedback. [For the complete volume of short papers, see ED572005.]