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.