Difficulties in the construction of equations when solving word problems using an intelligent tutoring system. We present results of a research conducted with secondary students (15--16 years old), which addressed the effects of the teaching of algebraic solving of arithmetic-algebraic word problems by using the software Hypergraph Based Problem Solver (HBPS). We show students’ performances in which they construct equations by making one of the unknowns appears isolated in one side of the equation. Students’ actions and comments suggest that this tendency stems from the difficulty in interpreting the equal sign as a comparison between quantities.