This dissertation presents a partial grammar of the Guipuzkoan dialect of Basque, and develops a theory of how to capture the syntactic and morphological relationships between arguments and argument-categorizing-elements (ACEs) in simple sentences. I assume the background of the theory outlined in A Syntax of Luiseno (Steele in prep.). Starting with the assumption that an ACE and an aux(iliary) particle sequence form together the sentential nucleus and represent the minimal requirement for sentencehood, I focus on the representation of the relations between the ACE-argument-structure and the morphological make-up of the aux form, that is, the aux-structure. I argue for a principle which determines whether the aux-structure satisfies the argument requirement of the ACE. In order to characterize overt case-marked constituents, I consider various syntactic phenomena. Then, I introduce and discuss (i) a rule which co-indexes directly an overt constituent with an argument-slot in the ACE-argument-structure, and (ii) a checking device which associates, on the basis of case-matching, an overt constituent with a morpheme (understood as a form plus meaning combination) in the aux-structure. Finally, I investigate the linear distribution of overt case-marked constituents. Special attention is given to the issue of focus in positive and negative simple sentences.