The purpose of this paper is two-fold. It aims at a general exploration of the auxiliaries that mark tense/aspect in Seediq from a grammaticalization perspective on the one hand, and on the other hand, argues that the future-marking auxiliary maha and the verb musa ‘to go AF’ are cognates. The co-existence of maha and musa is the result of a split of the verb *muSa due to the grammaticalization of maha as an auxiliary to mark futurity. The finding that Seediq musa and maha are cognate with the PAN *kuSa ‘to go’ implies that a competition change between *S > s and *S > h might have occurred in the Proto-Atayalic as well as in the PAN, a finding that is historically significant.