Why did Medieval Latin use "ad" with the accusative instead of just using the dative?

Question

Part of Documents of Medieval Latin (page 14) states several differences between Classical Latin and Medieval Latin. One is

• an increased use of prepositions where Classical Latin used a simple case of the noun, in particular the use of ad and the accusative instead of a simple dative, and in with the ablative in expressions of time instead of the simple ablative.

I've tried writing in this way, and it seems weird. Using a preposition with a direct object instead of just sticking with a lone indirect object doesn't make much sense to me.

For example, while Classical Latin writers might have used

Puerô pecûniam dat.

Medival Latin writers might have used

Ad puerum pecûniam dat.

The translation is roughly the same:

S/he is giving money to the boy.

But the use of a preposition seems totally unnecessary, and a bit unintuitive. So why did this convention arise during the medieval period? My completely naïve hypothesis is that there was some influence from Vulgar Latin around this time (which there was) that may have messed things up, but I'm not familiar with either that or this convention.

Answer 1

I'm not sure there is a "why", but it probably happened under the influence of contemporary vernacular languages and/or Vulgar Latin, since French, Italian, and Spanish also use more prepositions than Latin. This is probably also what steered Latin towards using quod instead of the accusative with infinitive.

Around the same time, I believe cases began to disappear, as can also be seen in the Romance languages. It makes sense for case endings and prepositions to compete with each other: if you don't have cases any more, you need prepositions to express e.g. the indirect object. Or the other way around: if you use more prepositions, you don't need cases as much any more. I don't know which influenced which, or whether it was a complex reciprocal development.

Answer 2

The problem with Latin grammatical cases is that, they are generally ambiguous between different cases. plural nominative masculine is the same with singular genitive masculine for example. Or singular dative feminine is the same with plural nominative feminine and singular genitive feminine.

So the ambiguity makes it complex, very hard to learn for rest of the empire that started "talking" Latin, meanwhile the natives just pattern match without even realizing, solving daily puzzles intuitively. So I guess new learners started cheating by using "this", "that" with prepositions instead and not declining. People coming from languages without genders or articles also find very innovative ways of cheating not to use or decline them while learning languages like German for example.

My mother language is a language that still has grammatical cases, and I think they survived to this day because there is no ambiguity and it's much simpler to decline. Most of today's languages are spoken by people that adopted them, we might all be innate cheaters.

Using the "ad" instead of the case makes it clear to the listener that it is dative form.

For example

Pulchrae puellae pecuniam dant

Is it

The beauties give money to the girl

Or

The beautiful girls give money?

Maybe

The beauties of the girl give money?

When you add ad things are clear.