This article, the fifth on the subject of Pashto preverbs, deals with the perfective preverb wə́-. Pashto wə́- cannot be studied separately from aspectual oppositions: in fact, wə́- characterises the “perfective” of simple verbs. Therefore, a quick review of the aspect in Pashto follows. Before turning to the etymology of wə́-, the uses and semantics of Persian bi-, with which scholars universally associate Pashto wə́-, are presented. The conclusion of the analysis is that *wi is the best candidate as the origin of Pšt. wə́-, for several reasons that are presented in detail in the article: in this way, the path of adverbs can be traced, passing with a first grammaticalisation to preverbs and with a second grammaticalisation to support for the aspectual distinction. From an etymological point of view, prosodic features play the main role and must be considered first in any kind of analysis.