There is only one way in which tense/aspect marking is reduced in negative clauses – negation is not compatibel with the past habitual marker yusdu~yustu. The special negative marker neba without tense marking is used in habitual past contexts (although it is not restricted to habitual contexts). Combinations of mood marking and negation were not investigated in detail.
Source: Angelo et al. 1998
Source: Sandefur and Sandefur 1982: 65
Source: Hodgson 1998: 196