Virtual manipulatives are part of a vast set of technological tools, which may be used in the teaching of Mathematics by the teacher in the classroom. The use of virtual manipulatives can provide interactive environments where students can make conjectures, formulate questions and solve their own problems, so as to establish connections between different mathematical con-cepts and operations, with immediate feedback of their actions. Algorithms of arithmetic opera-tions are an unavoidable topic in the early school years. A future Basic Education teacher has to understand the procedures involved in algorithms. In particular, the basic addition algorithm involves key concepts, such as place value, which is one of the basics of the principles of the system of decimal numeral system. Understanding the reasoning behind the algorithms is a very efficient way of acquiring many other essential mathematical skills. This study was conducted with 3rd year students of a Basic Education Degree Course in two sessions focused on the reso-lution of a group of tasks, which involved both the approach of the addition algorithm as well as the meanings of addition. Tasks were solved with the help of the Base Blocks Addition applet, included in the National Library of Virtual Manipulatives repository, while trying to understand its role in the understanding of the procedures related to the algorithm and which knowledge gaps the applet would lay bare. In doing so the aim is to answer the research question: deter-mine to what extent the use of this applet, during the resolution of mathematical tasks related to the basic addition algorithm, improves the understanding of pre-service teachers about the con-cepts and mathematical procedures related to this algorithm. Preliminary results show that dur-ing task resolution pre-service teachers have not always resorted to the most adequate mathe-matical terminology and that students have faced difficulties in terms of knowledge of the mathematical contents involved they were not initially aware of. However, their applet-assisted work allowed them to improve both the terminology used, thus avoiding the systematic reoccur-rence of expressions related to mnemonics, as well as the understanding of the principles of algorithm procedures, specifically, the composition in a unit of higher order.
Keywords: teacher training, virtual manipulatives, applets, addition algorithm.