From the introduction: "This study presents a relatively detailed description of the grammatical categories of mood and aspect in three Mixtec languages. Our goals include morphology in a broad sense, covering both meaning and form. Our focus is a grammatical category known in Mixtec studies as 'aspect', which distinguishes three forms of every verb, generally called completive, continuative, and potential. We present a view of the system that differs somewhat from previous work, suggesting that the system is primarily modal, not aspectual in character. Partly, this is just a matter of how terms like 'mood' and 'aspect' are defined, but there is a more fundamental difference. We would like to suggest that the system is best analyzed in terms of two binary divisions, one between realis and irrealis mood, and a second within realis mood between perfective and imperfective aspect. We believe this leads to an insightful understanding of the meaning and usage of these categories, and also of their formal expression, inasmuch as it mirrors their semantic structure.
"In the process, we are also able to give a first description of this system in three previously unstudied Mixtec languages. Our perspective is a comparative one, concentrating on those characteristics that are common to all three languages, and especially those which are found throughout Mixtec."
Bickford, J. Albert and Marlett, Stephen A.
"The semantics and morphology of Mixtec mood and aspect,"
Work Papers of the Summer Institute of Linguistics, University of North Dakota Session: Vol. 32, Article 1.
Available at: https://commons.und.edu/sil-work-papers/vol32/iss1/1