The presence of the subject pronoun becomes mandatory in these cases:
A) When the ending of the verb can apply to several people (in the subjunctive mood).
- Desideri che io (tu/lui/lei) ci cada?
- (Do you want me (you/he/she) to fall for it?)
- Pensare che io (tu) fossi arrabbiato con lui.
- (To think that I (you) was angry with him).
B). With indefinite moods (gerund, participle, infinitive).
- Arrivato lui, la festa cominciò subito.
- (Once he arrived, the party began immediately).
- Tu dire queste cose! non ti vergogni?
- (You say these things! Are not you ashamed?)
- Noi pensare questo di te!
- (We think this of you!)
C). When the pronoun follows the verb or is particularly highlighted.
- Sei stato tu a decidere la data, non sono stato io.
- (You decided the date, it wasn’t me).
- Lui sì, che ha capito!
- (Yes, he understood!)
D). When the verb is not expressed, but implied.
- Chi è stato? – Lui, non io!
- (Who did it? – Him, not me!)
- Contento tu. contenti tutti!
- (Happy you. everyone is happy!)
E). When the pronoun is accompanied by:
anche (also), pure (as well), nemmeno (neither), neanche (nor), neppure (not even), stesso (same), magari (maybe), addirittura (even), almeno (at least), non meno (not less), etc.
- “Se ti inviterà vermi pure tu?” “Si, tent anch’io”
- (“If he invites you worms too?” “Yes, I’ll try too”)
- “È lui stesso che insiste perché veniate anche voi“.
- (“He himself is insisting that you come too”).
F). When the subject pronouns are in opposition or comparison with each other.
- lo preferisco il mare, lei la montagna.
- (I prefer the sea, she prefers the mountains).
- Lui è arrivato stanco e trasandato, lei fresca e riposata.
- (He arrived tired and unkempt, she arrived fresh and rested).
G). When there are multiple actions with different subjects.
- Tu andrai manti, noi seguiremo distanza e lui ci aspeneni alla stazione.
- (You will go to Manti, we will follow the distance and he will drop us off at the station).