The nature of the software requirements is subjective and varied. For this reason the level of complexity increases according to the volume, especially when the requirements are made in a natural language. Therefore obtain quality software requirements that are understandable and unambiguous in the Spanish language becomes a necessity. First, a controlled syntax was proposed to express software requirements taking into account the static and dynamic behavior among the different actors of the system, where the expressions are elaborated based on the Backus-Naur form (BNF). Then a set of writing rules were adapted to the Spanish language, creating four additional rules. Finally, the results of the case study had high accuracy in understandability; also the ambiguity of requirements elicitation was reduced. In addition to improving the development of software engineering activities, since there are no tools available for the elicitation of software requirements with language Spanish.