12.4.9 exercise 9: The "si passivante" in the compound tenses
In the case of the si si passivante the verb essere is really justified, at least from a grammatical point of view because in this case si is a real reflexive pronoun and reflexive pronouns are conjugated with essere. The verb must match in gender and number with what would be the direct object in the sentence in active voice.
(One sees the house. <=> The house is seen).
exercise 9: Translate the following sentences with the "si passivante".
The cars have been washed.
Many books have been read.
Many things have been bought which were not needed.
All houses were sold because there were no work in that village.
All lot of useless things are being learnt at school.