Close analysis of Early New Persian texts shows that the ENP verbal prefix bi- (Modern Persian be-) had a directional sense and aspectually delimiting force, and was, in fact, a preverb. Crosslinguistic parallels are examined. Arguments that the occurrence of ENP bi- depends on word and phrase stress are shown to be invalid. The preverbal status of ENP bi- explains its development into ModP be-, a marker of the subjunctive and imperative. Only the assumption that ModP be- was originally an aspect marker can explain certain peculiarities of its present-day syntax.