So είπες is one of the past forms of λέω? And έφαγα is a past form of τρώω? Είδες of βλέπω? What??
Many Greek verb stems follow a set of rules when changing into past tense, but there's a bunch that have to be rote learned - they just do their own thing. I think they correlate to what was the 'strong' aorist in ancient Greek, where entire stems would change on the regular. Or like go/went in English.