First, are there any other cases in a Romance language where the main verb must agree with its direct object in number, when the object comes AFTER the verb? For example, La mattina si beve il caffè, but La mattina si bevono il caffè ed il tè. Are there any other linguistic constructions, impersonal or otherwise (it was unclear in my original post about whether I was making a direct comparison to French) wherein the verb agrees in number with its D.O., without the D.O. coming before the verb?
Second, although the impersonal construction is formed by si + third-person singular or plural of the verb, when a reflexive verb is used, the reflexive pronoun is ci, the first-person plural pronoun. For example, Quando fa freddo ci si mette una giacca. My professor said she guessed that this developed because the third-person S/P pronoun is si, and constructions such as "si si mette" sounded bad, so the sound changed into "ci." Does anyone know for sure?
First question edited for clarity.