(By Matthew S. Dryer): "This map shows the position of tense-aspect affixes. The primary distinction is between tense-aspect prefixes, as illustrated by the past tense prefix ā- in (1a) from Anywa (Nilotic; Ethiopia and Sudan), and tense-aspect suffixes, illustrated by the imperfective suffix -n in (1b) from Harar Oromo (Cushitic, Afro-Asiatic; Ethiopia).... No attempt is made here to distinguish tense from aspect, and it is frequently difficult to determine from descriptive grammars whether a category ought to be considered tense or aspect. Different descriptions of the same language often differ in whether they characterize a category as one of tense or as one of aspect. ❡
While the major types of morphological indicators of tense-aspect are prefixes and suffixes, there are three less common morphological ways to indicate tense-aspect: tone, infixes, and stem changes. Tone is the primary method in 13 languages on the map..."