We present the design principles for a new kind of computer system that helps students learn algebra. The fundamental idea is to have a system based on the microworld paradigm that allows students to make their own calculations, as they do with paper and pencil, without being obliged to use commands, and to verify the correctness of these calculations. This requires an advanced editor for algebraic expressions, an editor for algebraic reasoning and an algorithm that calculates the equivalence of two algebraic expressions. A second feature typical of microworlds is the ability to provide students information about the state of the problem in order to help them move toward a solution. A third feature comes from the CAS (Computer Algebra System) paradigm, consisting of providing commands for executing certain algebraic actions; these commands have to be adapted to the current level of understanding of the students in order to only present calculations they can do without difficulty. With this feature, such a computer system can provide an introduction to the proper use of a Computer Algebra System. We have implemented most of these features in a computer system called aplusix for a sub-domain of algebra, and we have done several experiments with students (mainly grades 9 and 10). We had good results, with positive feedback from students and teachers. aplusix is currently a prototype that can be downloaded from http://aplusix.imag.fr. It will become a commercial product during 2004.