I'm interested to know how hard it is for native Italian speakers to read The Divine Comedy in Dante's original language. The work was composed in the 14th century, so I imagine that there would be some difficulties. But I have no idea how well Italian was preserved between the 14th century and current times. Is the grammar mostly the same? The vocabulary? Would someone fluent in Italian be able to make out most of it?
Yes Italian speakers can read the Comedy. It usually requires some notes here and there, both due to the archaic language and to the historical references ( many people are likely to be unfamiliar with) but it can be done. In general the Inferno is the easiest part and the Paradiso is the hardest. The Comedy is usually read, at least in part, in high school.
Note that a large part of the difficulty comes from the fact that it is poetry rather than from the time difference. Boccaccio's Decameron, a work of more or less the same time period, can easily be read by educated Italians with only some notes to explain the more outdated references (similarly to how native English speakers can read Shakespeare's plays even if the language is not quite the same, what with all the thous and thees). To give you a sense of perspective of how little the language has changed, the Decameron was one of the inspirations for Chaucer's Canterbury Tales.
I'm interested to know how hard it is for native Italian speakers to read The Divine Comedy in Dante's original language
Reading and understanding each word is relatively easy, not much less than a poetry from a century ago. I'd venture that about three-fourths of the words are still recognizable and have the same meaning.
Some did change in subtle ways; for example però currently is used as a synonym of ma: "È brutto, però è tanto ricco!". In Dante's times, that phrase would have meant that a person's ugliness was the reason of his richness: però means perciò, per questo, for this reason. "Trasumanar significar per verba non si porìa: però l'essemplo basti" - one can't explain transhumanization with verbal means, therefore let the example suffice.
Or the form of the verb credo, nowadays used in the dubitative form, "credo che pioverà" = I think, I suspect it might rain, was then used literally: "credo ch'io vidi" = I firmly believe, I know that I saw.
It is not however a great obstacle to enjoying the poetry and the literal meaning of the work.
But Dante's poetry, increasingly from Hell to Paradise, becomes fraught with double and triple meanings that were difficult to interpret even then (the Comedy itself in its entirety is a metaphor - an anagoge). Dante himself says, O voi ch’avete li ’ntelletti sani, mirate la dottrina che s’asconde dietro ’l velame de li versi strani: o you of sound intellect, behold the doctrine hidden behind the veil of strange verses.
Inside the Comedy there are references to physics, mathematics and geometry that require explanatory notes (these were actually difficult to grasp at the time, and much easier for us): o se del mezzo cerchio far si pote triangol sì ch'un retto non avesse is a reference to one of the circumference theorems. At the beginning of the Paradise Dante refers to the law of refraction, and soon after Beatrice in Canto II instructs him in detail on how to perform an experiment on the intensity of reflected light.
Nowadays most of the references would escape the casual reader. "Nel suo profondo vidi che si interna ... ciò che per l'Universo si squaderna", literally means that in the depths of God's light Dante sees the unity of That that expands then in the whole Universe. But interna also meant becoming three, and it's a reference to both the Trinity and the three teologal virtues, the virtues that relate to God; just as squaderna also means expanding into four, as the four cardinal virtues that relate to Man. So this vision is also a theological reaffirmation of Christian doctrine. This was an easy reference back then1.
(1) It was suspected, even not so long after (e.g. by Christopher Landino and others) that many other references (e.g. raia da l'un, se si conosce, il cinque e 'l sei) had further meanings, and some even came to believe that the Comedy might hide secret messages for nothing less than a secret society to which they maintain Dante must have belonged.
As answered above by Denis, reading it for an Italian native is not much concern. The very concern stands in understanding it, even for a native, for several different reasons:
- The work is sublime poetry
- It's plenty of references: religious, philosophical, historical, astronomical which makes it very hard without a solid preparation.
To make an example, if reading a prose book for a native is as easy as walking,
reading the Comedy is like diving in the ocean where without study, effort, and a good annotated version even a native can drown.