@ Dan Smith:
classical Latin is different from popular Latin in several aspects. The most important is just the pronunciation: yes, it is possible to know how they pronounced the words in many way. One of them is studying onomatopoeic sounds, another is studying the assonances between different words, another one is observing the period in which words have entered into other language leaving Latin; for example 'Kaiser' left 'caesar' before 'cesare' did, this is an evidence that 'C' was spelled like 'K' rather than 'Tʃ'...
Another difference is about vocabulary, of course Cicero never could write "de motobirota sua", among the centuries many words, which were unknown to classical authors, have entered Latin vocabulary.
Last difference is about sentences construction and meaning of verbs, for example the announcement given when a new Pope is elected: "Habemus Papam", in classical latin would be "Nobis Papa est"