Categories covering the expression of grammatical information such as aspect, negation, tense, mood, modality, etc., are crucial to the study of language universals. In this study, I will present an analysis of the syntax and semantics of these grammatical categories in Lakota within the Role and Reference Grammar framework (hereafter RRG) (Van Valin 1993, 2005; Van Valin and LaPolla 1997), a functional approach in which elements with a purely grammatical function are treated as ´operators`. Many languages mark Aspect-Tense- Mood/Modality information (henceforth ATM) either morphologically or syntactically. Unlike most Native American languages, which exhibit an extremely complex verbal morphological system indicating this grammatical information, Lakota, a Siouan language with a mildly synthetic / partially agglutinative morphology, expresses information relating to ATM through enclitics, auxiliary verbs and adverbs, rather than by coding it through verbal affixes.