Abstract: Techniques are described for an automated grammar teaching system that displays sentences and allows a user to identify comma errors within the sentences, if any. The sentences may be presented as a set of sentences or a paragraph. When presented as a set, the sentences may employ similar or contrasting sentence structures to emphasize the application of the comma rules. The user may be asked to determine whether the sentences are correct or incorrect, to identify the locations of missing or extraneous commas, and in a sentence with a coordinating conjunction, to determine whether the conjunction joins two independent clauses. To guide the user, multi-level hints may be provided on request, and an incorrect user response may trigger the display of remediation information or a just-in-time hint. New sentences in the teaching system may be selected based on historical data maintained for the user.