We present an analysis of students' formal constructions in mathematics regarding to syntactic, semantic and pragmatic aspects. The analyzed tasks correspond to students of the Course of Mathematics for the admission to the university. Our study was qualitative, consisted in the identification, analysis and interpretation, focused in logic features, of the students' oral and written responses. Here we refer particularly to the following formal constructions: a) the decision of if an object belongs or not to a defined class, b) the determination of the value of truth of a composed proposition, c) immediate inference, d) analytic deduction and e) the refutation.