How did Vulgar Latin evolve from Classical Latin. Did Classical Latin evolve into Vulgar Latin or did they evolve in parallel? And which version is the mother tongue of the Romance Languages?
The truth is that scholars do not entirely know. For one thing, Only a few inscriptions have been found in Vulgar Latin mostly from Italy, followed by Gaul and North Africa. It would help if more were found. Inscriptions are to a historical linguist what ape-man bones and foot prints or dinosaur fossils are to a paleontologist.
We do know, however, that in the Ancient World Greek and Latin were living, changing languages just like English and Spanish are today. There was no permanent, fixed way to speak Greek or Latin and they were continually changing every few generations except in the most distant and isolated places like Britain, North Africa, Sardinia and (Dacia) Romania. For example, in Britain and North Africa, Vulgar Latin speakers were still using PULCHER / PULCHRA for "beautiful" just as in Classical Latin; but Vulgar Latin speakers on the European continent were using other words like FORMOSU / FORMOSA and BELLO / BELLA.
The Latin that Aenid, Cicero and Julius Caesar wrote in is generally called "Classical Latin" and even St. Augustine"s Latin from around 400 A.D. in North Africa is still considered "Classical Latin" albeit a peculularly African-typeClassical Latin which specialists in European Classical Latin have some difficulty reading.
During all this time however, the slaves working in the kitchens, the barber shops, the hair salons, the vinyards, the olive plantations and the silver mines were speaking a racy slang Latin that would eventually develop into modern day French, Spanish and Italian; but their Latin was never recorded or written down or recorded.
***One African Vulgar Latin inscription that has been found reads Ut facia lum mortu "May he be killed". In proper (Classical) Latin it would have been Ut faciat illum mortus. The subjunctive particle "ut" tells us that North African Vulgar Latin was archaic because in Europe they were using the word quae which later became Spanish, Portuguese and French que and Italian che.
Of all Romance languages spoken today, Romanian is the only language that has preserved something of the Latin case system, although it was also modified in the process (e.g. the plural genitive inflection "-ilor" or "-elor", depending on gender, derives from the Latin inflection "-orum" or "-arum", cf. "timpurilor" vs. "temporum", meaning "of (the) times"). All other Romance languages have become completely analytical, using prepositions instead of a case system, although some of those prepositions were already used with Latin inflected nouns, but with a slightly different meaning (e.g. "de" with ablative case used to mean "of" or "about" before becoming the preposition indicating possession or belonging, as Italian "di" or French and Spanish "de"). The definite article "il", "le" or "el" derives from the indicative pronoun "ille, illa, illud", etc. I guess this took some centuries to happen, but it is not really possible to tell when it started.
I agree with Easterner on the above. Although Classical Latin had no definite article, Late Latin and Vulgar Latin (they kind of overlapped with each other) acquired one based on 'ille' which originally meant "this here". Thus, illum librum "This here book" would eventually become Spanish el libro, Portuguese o livro, Catalan el llibre, French le livre and Italian il libro all meaning "The book.
Librum was lost in a few areas. Romanian has carte for "book" from another Latin word (of Greek origin) charta which originally mean "a sheet of papyrus". This word is also the source of the English word "charter". Engadine on the Swiss-Italian border has codesch from codex which originally meant "a wooden tablet". .
Except for "ipse" "ipsa",which gave the article in Sardinian, a few Gascon dialects and Northern Eastern Coast and Balearic Isles Catalan dialects. In Balearic Catalan they still say "es pa" and "sa ma" (the bread, the hand) instead of General Catalan "el pa" and "la mà".
Did Spanish words evolve from the Nominative case or the Accusative case?
Sometimes it's obvious to think that the Nominative case was the origin of Spanish words (Nom. case "arena" -> "arena" in Spanish, the same word), but what about "hombre"? It may come from "hominem" rather than "homo", don't you think?
Xatufan: More often than not, words came from the Latin nominative case in French, Spanish and Portuguese but the accusitive case in Italian and Romanian. Compare Spanish / Portuguese cruz, luz, paz "cross, light peace" with Italian / Romanian with cruce, luce, pace from accusative forms ending in -em.
Jordi: It's good to mention exceptions in the definite article for dialectal Catalan and Sardinian.
Had Vulgar Latin survived in North Africa or Britain it is likely that the Sardinian system would have been used in any Romance languages that developed there since professor Stephen Omeltchenko tells us that the Latin spoken in these regions was of the archaic Sardinian qualitative voice type.
accusitive = accusative (correct spelling).