I fell upon this form in Herodot, 6,107, ἀπέβησε and I can’t see what else it could be than aorist preterite of ἀποβαίνω, but how come Herodotos, a most classical writer, incorporate such a display of analogy? In another place, in the first book somewhere, I also saw εἰπας for εἰπων, and it’s really baffling me.
Are analogies more common in Ionic or what? Am I on the wrong track?
βαίνω has both first and second aorist forms. When a verb does this the second aorist is usually intransitive and the first aorist is transitive. Aorists like ἔβησα even occur in Homer.
but how come Herodotos, a most classical writer, incorporate such a display of analogy? In another place, in the first book somewhere, I also saw εἰπας for εἰπων, and it’s really baffling me.
There cannot be Koine in Herodotus because it didn’t exist yet. However, Koine did adopt some features of Ionic, which included some analogical simplifications, such as οἶδας for οἶσθα, etc.
What I meant was koine-isms, or simplifications by analogy. I feel terrible whenever it seems I don’t know something I in fact know very well, for example when koine was spoken/written.
I see your point now, and I should have looked more thoroughly into it as I have now found the same explanation in one of my grammars, as well as the possibility that ἔβησα can also be understood to be aorist of βιβάζω. What in the first place struck me was that the so-called first aorist in this case was formed on the root-aorist stem, and not the present stem, as first aorists tend to do.
Don’t worry about it. When I can I try to answer questions for the broadest likely audience. People read these posts well after the original author forgets them.
I see your point now, and I should have looked more thoroughly into it as I have now found the same explanation in one of my grammars, as well as the possibility that ἔβησα can also be understood to be aorist of βιβάζω.
Hmm. That seems over-nice to me, since both βαίνω and βιβάζω are formed of the same βα- root.
Now that I think about it — if you can find a copy in a local library, Amy Barbour’s Selections from Herodotus has a great 40-or-so page summary of Herodotus’ Ionic. Even if you don’t want the whole book, photocopies of that summary might be useful.
βαίνω and βιβάζω are two different lemmata in the dictionary, though 
That’s regretfully the way in which I tend to define what ‘a word’ is.
I think (but I’m not sure) that the present form βιβάζω exists only in some dialects, viz. Attic, as a derived verb-form via reduplication of the βη-/βᾰ stem to show causativity: β-ι-βά-ζω, “to cause something to go/move”. cf. πί-μ-πλη-μι “to cause something to be full (πλη)”
other dialects, including Ionic, use the same present verb-form (βαίνω) for both the intransitive (“to go”) and the causative (“to cause to go”) meaning.
in all dialects, though, these two meanings always differ morphologically in the aorist: ἔβησα “I caused to move” vs. ἔβην “I moved”.
it is not unusual that the same verb-form has an intransitive and a causative meaning. the case of ‘Ionic βαίνω’ may be parallelled by English to change: The times have changed, but also I changed the baby’s diapers