How do perfective and imperfective verb stems work in Nilo-Saharan languages?

In Nilo-Saharan languages, the verb is often the most complex grammatical component, frequently involving extensive marking for categories such as person, number, tense, and aspect. A widespread and stable feature across diverse groups—including the Saharan, Taman, Nyimang, and Surmic languages—is the formal distinction between perfective and imperfective aspect verb stems.1

While these languages utilize this aspectual distinction, the specific morphological processes involved often include consonant mutation to mark these conjugational features. This system of aspectual stems is a distinct characteristic of Nilo-Saharan verbal morphology and should not be confused with the vowel-based stem patterns found in other language families, such as the Semitic languages.12