Rule reminder
Verbal periphrases encode ASPECT — how an action unfolds in time: INCEPTIVE (start): cominciare a, mettersi a, iniziare a; CONTINUATIVE (ongoing): continuare a, stare + gerundio, andare + gerundio; TERMINATIVE (end/stop): finire di, smettere di; PROSPECTIVE (about to happen): stare per, essere sul punto di; HABITUAL (regular): solere, essere solito/a. Choosing the wrong aspect changes the meaning entirely.
'___ uscire quando ha telefonato.' (was about to leave when he called) Best form?