Herbert Weir Smyth, A Greek Grammar for Colleges

Thus, ὑπερβαίνω pass over, ὑπερέβαινον, ὑπερβέβηκα; εἰσβάλλω throw into, εἰσέβαλλον, εἰσβέβληκα.

a. Before ε of the augment ἐκ regains its fuller form ἐξ (133 a), and ἐν and σύν reappear in their proper forms which were modified in the present. Thus ἐκβάλλω throw out, ἐξέβαλλον, ἐκβέβληκα; ἐμβάλλω throw into, ἐνέβαλλον; συλλέγω collect, συνέλεγον, συνείλοχα; συρρί_πτω throw together, συνέρρι_ψα, συνέρρι_φα; συσκευάζω pack together, συνεσκεύαζον, συνεσκευάσθην.

b. Prepositions (except περί and πρό) drop their final vowel: ἀποβάλλω throw away, ἀπ-έβαλλον; but περιβάλλω throw around, περιέβαλλον, προβαίνω step forward, προέβην. But πρό may contract with the augment (προὔβην).