Rule reminder

Common errors with Italian impersonals: (1) using 'si si' instead of 'ci si' for reflexive impersonals; (2) wrong adjective agreement with si impersonale (singular instead of plural); (3) wrong auxiliary in compound tenses; (4) adding 'di' after 'è necessario/importante' before an infinitive; (5) using indicative instead of subjunctive after 'è opportuno che', 'è necessario che'.