This paper presents the design of Forge , a tool for teaching formal methods gradually. Forge is based on the widely-used Alloy language and analysis tool, but contains numerous improvements based on more than a decade of experience teaching Alloy to students. Although our focus has been on the classroom, many of the ideas in Forge likely also apply to training in industry. Forge offers a progression of languages that improve the learning experience by only gradually increasing in expressive power. Forge supports custom visualization of its outputs, enabling the use of widely-understood domain-specific representations. Finally, Forge provides a variety of testing features to ease the transition from programming to formal modeling. We present the motivation for and design of these aspects of Forge, and then provide a substantial evaluation based on multiple years of classroom use.