Like περι-, προ- preverb keeps its final vowel when added to a radical as in "προ-αιρέω". But my French->Ancient Greek dictionary, the old Bailly, tells me that προβάλλω becomes either προέβαλον either προῦβαλον at the aorist 2. I can't find any further explanations in my grammar books : why such a contraction ?
Can someone give me an explanation about these two forms ? Do the vowels contractions after προ- depend on the phonetic environment or is it something linked to language evolution ?
NB : same remark with προκόπτω > imperfect προύκοπτον in this extract but > προέκοπτον in Galatians 1.14 .