Most software tools that have been developed with the aim of helping students to learn school algebra have not yet achieved successful results in classroom. Almost all of them are menu-based systems that provide transformation rules in menus and buttons. Aplusix is a new interactive software tool for learning school algebra. In contrast to existing software tools in mathematics education, Aplusix has been developed to allow students to freely build and transform algebraic expressions as they can do on paper. In addition, Aplusix provides appropriate feedback and interactivity, and as such, it becomes a source of learning. This work reports on a research project on the investigation of students' learning of elementary algebra with Aplusix. The work uses the Theory of Didactical Situations (TDS) to analyze and evaluate the learning potentialities of Aplusix. The paper also reports on implications for the learning of school algebra and the integration of Aplusix into mathematics classroom. (Contains 6 figures and 2 tables.)