Nuosu Yi is typologically exceptional as it is a classifier language with an overt definite determiner, unlike most other classifier languages (Jiang 2012, 2018, 2020). In Nuosu Yi, the determiner (D) cannot combine directly with the head noun and a classifier (CLF) is required, resulting in the N-CLF-D linear word order. This paper proposes a morphosyntactic analysis to account for the strict CLF-D word order in Nuosu Yi, based on two key observations. First, Nuosu Yi can express definiteness with [N-CLF] phrases without D, but only when the classifier undergoes tone sandhi. Second, D can also function as a quantificational determiner (Q-det), in which case there is no tone sandhi on the preceding classifier. Specifically, I argue that the CLF-D order in Nuosu Yi is derived through CLF-to-D head movement, accompanied by the formation of a [CLF-D] complex head. To capture the dual roles of D, I further propose that D is ambiguous between [+definite] and [+distributive]; in syntax, the classifier undergoes head movement to D to value the [+definite] feature on D. At PF, the syntax-semantics feature [+definite] could trigger a tonal change on the preceding classifier.