The purpose of this work is to determinate whether or not a method of subjunctive mode existed in Preclassic French, and in the affirmative, to describe and to explain it. From a morphological point of view, we notice Preclassic French was the age of simplification and analogy, while medieval system was marked, on the contrary, by a tendency to differentiate the subjunctive’s forms in a characteristic way. Besides, the presence of the yod at first and second plural persons becomes widespread, expressing in a better way the indetermination and “desactualisation” compared to the medieval system devoid of the yod. Regarding the syntax of the subjunctive mode during this period, it is characterized by an incredible freedom: the only rule actually followed by the speaker consisted, in case of doubt, in using the subjunctive tense, the “world of the possible”, in opposition to the “world of the likely” which is specific to the indicative mode, both concepts belonging to Robert Martin’s terminology. All along the aforesaid historical period, it is particularly interesting to see to what extent the modal system remains free from restrictive grammatical regulations. Indeed, the subjunctive mode doesn’t obey mechanical rules, except some tendencies. It only obeys kinetic and modal concordances. Despite its autonomy, subjunctive mode was nevertheless frequently ruled out to the benefit of indicative mode, including the “forme en –rais”, especially when authors showed no interest in stylistic or rhetorical effects. In a matter of fact, while indicative mode gives a precise indication of the time, mental representation of the verb within the subjunctive has the distinctive characteristic to be incomplete and indivisible as well, qualified as “amorphe” and “in fieri” according to Guillaume’s terminology. However, despite its inherent temporal imprecision, writers of that time widely used the subjunctive mode, judged exceptionally poetic and therefore irreplaceable.