Suddenly, I spontaneously generated an example: Donabat mihi librum, "He gave me book." Not, "he gave me A book," nor "he gave me THE book"; just, "he gave me book." To give a sense of definiteness to this, you would have to say something like, Donabat mihi illum librum, "he gave me that book."
Now, the Germanic peoples who took over what had been the Roman Empire acquired Latin as part of their inheritance. Two things then began to happen, more or less simultaneously. In writing Medieval Latin, they began to use a lot of demonstrative pronouns, because they were all used to using articles in their Germanic languages, and their writing seemed to require them. Meanwhile, those who adopted the common Latin being spoken by the peoples they had taken over the rule of began to use those same demonstratives in an article-like way. Which is why French (L'etat) and Italian (Il Trovatore) and Spanish (El Cid), though directly descended from Latin, all have articles. Late classical Latin was probably already sort of heading that way, I think, but certainly the influx of people of Germanic linguistic habits all writing and speaking Latin as a learned language must have greatly speeded up the tendency.
Enough. I tried to get my brain back on what it was supposed to be doing, i.e., writing a newsletter piece. But then, I thought: how many people can spontaneously generate a Latin sentence forty-seven years after having left High School Latin behind?
Nerdly, nerdly, nerdly.