Within the theoretical framework that he presented at the XVIIIth Congress of Scholars of Romance languages held in Trier (1986), the author investigates the relation between conditional sentences (beginning with si ) and the modalities of sentences. The investigation is carried out from two different aspects. On the one hand, it investigates the relation of the word si to negative and affirmative modalities. Secondly, it investigates “substitutions for si “, i.e. those syntactic constructions that substitute for the conditional sentence starting with si , where we find interrogative and imperative sentences in the place of prolosis (Example: Restait-on dehors, on fondati au soleil. Fais un pas, je t'assomme. ) This complementarity points to a kind of “hypothetical (conditional) transformation" which - differently than the interrogative and imperative modalities - does not start with the virtual positive but with the sentence previously determined as affirmative or negative; the hypothetical (conditional) noininalizator si indicates to the affirmative or negative sentence : (Si ) -+ Il (n) etati (pas) là , and never to the interrogative, imperative or optative with which it, otherwise, shares the area of virtualizing modalities and constructions, located between negation (0) and affirmation (1).