In these 3 cases: a) the case cited by Juan. b) when the verbal voice and the speaker alone does not imply the subject. c) when the verbal voice and the speaker alone imply the grammatical subject, but not the individual. Example of (c) s1. Anna e Giovanna si sono viste, poi e` andata via di corsa dopo aver ricevuto una telefonata. The grammatical subject implied by /e` andata/ is surely /lei/, but the individual is still in ambiguity (sentence s1 is NOT correct). s2 (correct) s1. Anna e Giovanna si sono viste, poi Anna e` andata via di corsa dopo aver ricevuto una telefonata. When the subjects are of different genders it goes like this: s3. Anna e Marco si sono visti, poi e` andato via. Obviously (andatO) the speaker is talking about Marco, but still the sentence needs a pronoun, as grammar has been shaped following more general situations. s4. (correct) Anna e Marco si sono visti, poi {Marco, lui} e` andato via. === What about the verbal voice /sono/ ? It implies one of {io, loro}, but usually the pronoun is dropped for /io/ both in answers and in opening sentences, and is not dro[pped for /loro/ at least not in opening sentences. The context and the use of past-particle in the singular will imply /io/, in this case the grammatical subject uniquely identifies the speaker.

when you want to emphasize the subject.. Io sto lavorando, tu non. But it depends on the situation.

I see you speak *Brazilian* Portuguese....