12.3.3 particularities in the combination of si with the reflexive pronoun 2
personal pronouns and pronominal adverbs are in front of the si impersonale
Personal pronouns are words like I / you / he / she / it / we / they (nominative), me / you / her / his / us / them (object pronouns). The only function of personal pronouns is to represent something, nothing else. If si is an indefinite pronoun (corresponding to one in English sentence like One should know that) the personal pronoun is put in front of the indefinite pronoun (Lo si permete => It is allowed). But if the si is a reflexive pronoun, the personal pronoun follows the si.
examples
Se lo permette.
He allows it himself.
Lo si permette.
It is allowed.
Se la compra.
He buys it himself.
La si compra.
It is bought.
If si is combined with si, is not so complicated. Si si becomes ci si. But things can be more complicated, for instance it there is a personal pronoun as well.
Ce la si compra. <=> It is bought for oneself. (People buy it for themselves. )
You already know that the past participle has to agree in gender and number with a direct objects that stands in front of the verb complex.This doesn't change if there is a reflexive pronoun.
Ce la si è comprata.
It has been bought for oneself. (People bought it for themselves.)
In this case the è (essere) is even motivated because in contrary to sentences like "Si è parlato di questa storia", "It has been spoken about that affaire ", the use of the verb essere is justified, because it is a reflexive construction.
