What is the origin of the Germanic strong preterite optative endings, corresponding to Gothic <be:rjau, be:reis, be:ri, be:reima, be:reith, be:reina>, OHG <ba:ri, ba:ri:s, ba:ri, ba:ri:m, ba:ri:t, ba:ri:n>? The obvious source is the IE athematic present optative, which like most of the Germanic forms has zero grade, but I don't understand why a present-tense form should become a preterite form. Is there an explanation for this shift of tense, if indeed this is the true origin of the Germanic strong preterite optative?
Andrew