Latin's rules are very static and formal, which makes sense for a language preserved in liturgical contexts and in science writing up through the 18th century. I just wonder if a modern Latin speaking time traveler, who learned it in High School, would be inteligible to all Romans. Rome existed for a very long stretch of time. We can't understand English from 1000 years ago.
Whether or not any particular modern time-traveler would be capable of interacting with ancient and medieval Romans would presumably depend entirely on the quality and focus of their education, but 'we' certainly do in the abstract understand most things about Latin and its evolution over time. The earliest distinctive forms of what we today talk about as 'Latin' developed out of a dialect of Proto-Indo-European, alongside several other similar and geographically colocated languages of antiquity, collectively grouped in the Italic bucket. What is taught in schools today is largely the literary language ("Classical Latin") to which these archaic forms of Latin ("Old Latin") gave rise, as used after ~75BC.
A whirlwind tour of salient (and grossly simplified) Old Latin developments, from which Classical Latin, in no particular order:
Scan this table, which contrasts two masculine and feminine Classical Latin forms descended from the original Indo-European word for god, and a parallel Vedic Skt. masculine.
| Singular | Plural | |
|---|---|---|
| Nominative | dīvus, dea (devás) | dīvī, deae (devā́s) |
| Genitive | dīvī, deae (devásya) | dīvōrum, deārum (devā́nām) |
| Dative | dīvō, deae (devā́ya) | dīvīs, deābus (devébhyas) |
| Accusative | dīvum, deam (devám) | dīvōs, deās (devā́n) |
| Ablative | dīvō, deā (devā́t) | dīvīs, deābus (devébhyas) |
| Instrumental | (devéna) | (devaís) |
| Vocative | dīve, dea (déva) | dīvī, deae (dévās) |
| Locative | (devé) | (devéṣu) |
Per all the above, when reading Old Latin inscriptions, one finds forms like deivos and duonos*,* instead of their classical descendants (divus, bonus).
Old Latin also had a word-initial stress accent, which compressed the following vowels, so pater ("father"), but Iūpiter; datus ("given"), but praeditus.
The Basilica of San Clemente in Rome is one such site, where the subterranean church has original frescos ‘captioned’ with a mixture of Latin and early Italian phrases.
The site is an early Christian church near the colosseum - one which decorated its walls with religious imagery and in the 4th century was already using Italian writing in tandem with Latin.
I can’t address the linguistic side of its development but there is significance to see a birth of a new language inscribed on church walls, not 500 meters from the colosseum.