In the first two cases it's a different verb altogether that supplies the past tense:
βλέπω (same as in ancient Greek) --
είδα from AG
είδον, from AG root
Fid- (the same as Latin
vid-eo)
λέω or λέγω from AG
λέγω --
είπα from AG
είπον, originally
έFεπον, proto-Greek root
eweikʷon
In the third case it is the same verb originally, having taken various forms through time:
έχω from Proto-Greek hékʰō, from Proto-Indo-European
seǵʰo -- In the past tense formed with the usual
ektasis (an e- added in the beginning):
e(h)ekhon >
eikhon είχον
In the last case it is again another verb that forms the past tense:
έρχομαι (same as ancient Greek) --
ήρθα, or
ήλθα, from AG
ήλθον, originally from AG root
ελθ--, which gave also future tense ελεύθ-σομαι > ελεύσομαι (it's the same root that gave ελευθ-ερία, the act of coming and going freely).