Is reading early Medieval Japanese for a modern Japanese speaker like reading Chaucer for a modern English speaker? Ie, slow and head-scratching but otherwise can get the gist of it.
Chaucer's time was the 14th century. I don't know much about the Japanese from that time, but it happens that Project Gutenberg has a 1975 English translation of a 1632 text on the Japanese language, by Portuguese author Diego Collado, originally written in latin.
The book can be a tad hard to read, as all the pronunciations are given in Portuguese phonetics. But there are a number of example sentences in there, and I will grab a few of them and (attempt to) translate the Japanese from the book's peculiar phonology into more appropriate rо̄maji. The English translations are the book's, not mine.
The following, at least, would seem to be at pretty accessible to a modern reader:
- mizzu fitotçu nomitai mizu hitotsu nomitai I would like to have a drink of water
- anofito no vo toró ano hito no wo torou I shall take what belongs to that man.
- mairu mai to no dancó ni qivamatta mairu mai to no dankou ni kiwamatta it was resolved that he not go [modern Japanese would use mairanai and kimatta]
- xô tame no chôgui gia shou tame no chougi ja this is the plan (ars) according to which it will be done [chougi might be 朝議?]
- maitte nochi no dancó maitte nochi no dankou the consultation he arrived after
But on the other hand, there seem to be plenty of verb tenses that don't exist in modern Japanese. For the verb ageru (aguru in the book), aguru is given as the present tense, with agenu as the negative. So far so good (agenu would be recognized by a modern reader, though it clearly sounds old). But in addition, ageouzu is given as a "future" tense, with aguru mai as the negative future tense. The equivalent of ageru kedo seems to be given as ageredomo. The past tense of agenu is agenanda. I'd guess some of these at least might cause some headaches.
A section on pronouns lists watakushi, soregashi, ware, warera, mi, midomo, midomora, and ura as the first person pronouns; I'm guessing three of those are widely understood, and the rest not so much (though, given context, perhaps ura would be connected to oira or ore?)
On second person pronouns (translating from the Portuguese phonology:)
In speaking to inferiors there are three particles used for 'you'; ware, onore, and sotchi. If me or mega is added as in wareme or waremega it means we very much despise the person being spoken to. If we speak to people who are on our own level, or just a little inferior, we use one of the three particles sonata, sono hou, or waresama. If we speak to a superior person, or someone on an equal level but with whom we must speak elegantly, we use one of the seven particles konata, kisho, kihou, gohen, kiden, konatasama, and sonatasama.
I'd expect sonata, sono hou, and sotchi to be understood. I think about half of the rest would be incorrectly understood as first person (and of course some of them are also listed as first person), and maybe the rest is gibberish.
Apparently by this time, the standard mix of kanji and hiragana (majiri bun) would have been in common use... but the kanji would be the old, traditional Chinese forms (compare 会 with its old form 會, or 声 with 聲), and the hiragana too would look very different, far more cursive and stylized, and there was less standardization and more hiragana characters than exist today. So reading it as actually written might prove very challenging indeed. On the other hand, I'm not actually sure how the modern English reader might fare trying to read an actual contemporary manuscript of Chaucer either, in, what, handwritten, stylized blackletter and with the "long s" character, and a "y" used to represent "th" (replacing the older thorn character). But I'm sure we'd have the easier time.
Even without going nearly so far back as the 1600s, just taking written Japanese from the early 1900s would pose something of a challenge to modern readers I think (and of course, any of the things that make that period difficult will apply at least as well to much older Japanese). I've a number of books from that time; the word かいましょう was written かひませう, おんな was written をんな, いっています could be written いつてゐます. And the kanji characters were still being written in their old, traditional Chinese forms (see above). Jim Breen has a page with a decent overview of spelling differences from that time.