At one point, the Latin passive infinitive was formed with a suffix -(r)ier, as in agier "to be driven", amārier "to be loved".
Allen and Greenough call this an "ancient form[…] found chiefly in poetry", but it is indeed well-attested poetically: it shows up in Vergil, Ovid, Lucretius, Naevius, and others, as well as the older Latin of Plautus.
Do we know when this transition happened: when -(r)ier became archaic and poetic, and -(r)ī took over? To put it another way, if I went back in time to Ancient Rome, at what point would I not sound pretentious saying agier and amārier in place of agī and amārī?
