Stare + the present gerund is used to indicate an ongoing action. [present continuous)
- Le vacanze stanno trascorrendo senza incidenti.
- The holidays are passing without incident.
- Carlo stava mangiando quando bussarono all porta.
- Carlo was eating when there was a knock on the door.
This form is used almost exclusively with the present and the imperfect, rarely with the simple future, but never with compound tenses.
The use of the form stare + gerund is not obligatory. It is frequently replaced by the present [indicative/ subjunctive/ conditional] or the imperfect [indicative/subjunctive].
- Nevica da questa mattina. = Sta nevicando da questa mattina.
- It has been snowing since this morning.
The form stare + gerund is used almost exclusively with verbs that express an action, but normally not with those of feeling or opinion, essere, avere, modal verbs, etc.
- Il bambino stava giocando con il suo gatto, quando arrivarono i genitori.
- The boy was playing with his cat when his parents arrived.
Sto sapendo che Luisa torna il 30 agosto dalla montagna.- So che Luisa torna il 30 agosto dalla montagna.
- I know that Luisa returns on August 30 from the mountain.