Most verbs took one subjunctive suffix in Proto-Celtic, -(a)s-, followed by the thematic primary endings. It was a descendant of the subjunctive of an Indo-European sigmatic thematic formation *-seti. The -ase- variant originated in roots that ended in a laryngeal in Proto-Indo-European; when the *-se- suffix was attached right after a laryngeal, the laryngeal regularly vocalized into *-a-. It would then analogically spread to other Celtic strong verb roots ending in sonorants in addition to the weak verbs, even if the root did not originally end in a laryngeal.
