This paper aims to present the theoretical considerations and methodology used in elaborating a formal characterization of a fuzzy grammar in natural language grammars. It specifically focuses on the syntax of Spanish. However, we suggest that this methodology could be used to define a universal model for describing any kind of natural language grammar that takes into account fuzziness. Objective data based on frequencies were extracted using the Spanish Universal Dependencies Corpus Treebank and the Marsagram tool. These data allow us to describe Spanish Natural Language Grammar in terms of its constraints using the Property Grammars Theory. The work presented here could be applied in the form of an algorithm for parsing which could be of benefit to various areas of language and technology such as self-taught language learning software (in which violations and degree of violation could be tagged), data mining or human-machine interfaces.