First up, my vanity is well gratified!
Well, there’s the question, and then there’s the details.
Can I understand someone speaking modern Tsakonian, or read ancient Arcadian and understand it, sight unseen?
Mate, I struggled to understand the Cypriot of my cousin’s husband Fotis; and I have no idea what Homer is on about. Homer!
I’m a really bad example, because I’ve approached Greek as a linguist rather than a classicist, so I’ve learned only the bits I’ve needed. I know that when I was studying my thesis on Modern Greek dialect, I was familiar enough with Pontic that I could read it without a problem, and I probably could hold a conversation in Tsakonian. It’s patchier 20 years on. And I would still struggle with Cypriot basilect, or Samothracian.
Ditto Ancient Greek, and that’s exacerbated by my imposter syndrome. I can kinda understand Attic, but I will sneak peeks at the dictionary when I don’t think you’re looking, and I ain’t touching no Thucydides. I know the Doric shibboleths, so I can probably deal with the Doric in Aristophanes and Archimedes; maybe not Alcman and Theocritus. I did intensive work with Alcaeus and Sappho, so I’m better than the usual classicist on Aeolic. But, because the TLG lemmatiser already dealt with Homer and Herodotus, when I first obtained it as Morpheus from Perseus, I never needed to brush up on my Epic/Ionic.
And non-literary dialects? I’ve read the handbooks of Ancient Greek dialect, such as Thumb and Buck and Bechtel, so I’ve *seen* North-West Greek and Arcadian and Cretan. Understand them? I’d be struggling. I’d pick out a few words more than the average classicist, perhaps, but that wouldn’t be enough for me to do a translation viva.
Edward Conway brings up Linear B in comments, and I’m just going to pretend I didn’t hear him. 🙂
Now, to go to your details: can you triangulate dialects (let alone intermediate stages of the language) from Attic Classical Greek + Standard Modern Greek?
Intermediate stages: Usually. Dialects: Less so.
We don’t have as much Greek attested between Attic and Early Modern Greek as you might think, because most people tried to write Attic. (A very artificial Attic.) Koine is not really challenging if you know Attic; you’ll be relieved at the simplifications, and the occasional Doric-looking words won’t throw you. The papyri are as much Greek as a Foreign Language as they are Koine, but they won’t really throw you either. In between the papyri and Early Modern Greek, we have bits and pieces: snatches of songs, inscriptions written by Greek POWs under the Bulgars. Again, no problem.
Actual Early Modern Greek starts 1100, more or less. There are going to be some archaic words and grammatical usages that will throw you a bit more, if you’ve got just Attic and SMG, and you want to be on the alert for false friends. You’ll understand the gist of things, but you may miss the fine print.
When I co-translated a poem written in 1364 (An Entertaining Tale of Quadrupeds), we looked up every single word in the Early Modern Greek Dictionary, because there were a lot of words that changed in strange ways. The modern word for “pew” for example, στασίδι, was just the mediaeval word for “a spot”: the Rat went back to his spot in the assembly, not to a church pew. The future tense looked very different from Modern Greek, with the modern form originating only in the 1400s, and not really settling down until the 1700s; so you could be missing some nuance there. Prepositions also worked slightly differently.
But honestly, most of the difficulty you’ll find in Early Modern Greek will be dialectal, rather than chronological. If you’re going to read Early Modern Greek, you’re going to find a lot of Cretan material in particular. Dialects are often archaic in some ways, but just knowing Ancient Greek isn’t going to be enough to work them out.
As a little sampler: here’s one of the very few private unlettered letters we have preserved in Early Modern Greek, from 1420 Crete.
I’m curious how easy Greek Quorans—particularly those unfamiliar with Cretan dialect—find it to read.