Основа и флексия в индоевропейската и праславянската глаголна система

Due to the appearance of the new imperfective formation *-ěax- in Proto-Slavonic, the verb system became the basic medium for expressing temporal relations. That is why the old opposition between primary and secondary inflexions became redundant, the result being a partial neutralization of that opposition; cf. the Old Bulgarian inflexions for the third person singular, the first, second and third person plural and the dual.