"(Non) appena" essentially has a temporal value, a value which serves to underline the close succession of two actions (that of the temporal phrase and that of the main phrase) and which can be exclusive and therefore correspond to:
• subito dopo che (immediately after)
• quasi nello stesso momento in cui (almost at the same time when)
but which can also be used to express an eventuality, a hypothesis with a meaning similar to:
• quando (when)
• tutte le volte che (every time)
• se (if)
In the first case, therefore in temporal sentences that express real and not eventual actions and events, "(non) appena" requires the verb in the indicative:
– non appena arrivi, chiama (as soon as you arrive, call)
– appena potrò, verrò a trovarti (as soon as I can, I will come and see you)
– appena ebbi finito gli esami, andai in vacanza (as soon as I finished my exams, I went on vacation)
Another possibility for this type of sentence is the use of the past participle:
– appena entrati, presero posto in prima fila (as soon as they entered, they took their place in the front row)
with ellipsis of the auxiliary verb, an ellipsis which can affect the entire verb form when the verb is either "to be" or in contexts in which the unexpressed verb can be easily reconstructed from the sense of the sentence:
– appena (furono) dentro, si sentirono al sicuro (as soon as (they were) inside, they felt safe)
– appena (fu/si fece) giorno, partì (as soon as (it was/came) day, they left)
In the second case, that is, when "(non) appena (che)" introduces sentences that convey an eventuality with semantic priority with respect to the temporal indication, it will be necessary to follow the conjunction with a verb in the subjunctive, while the main (regent) phrase will have the verb in the conditional to indicate the posterior nature of its occurrence with respect to that of the temporal phrase, according to the normal construction of the hypothetical clause of the eventuality:
– (non) appena l’avesse visto, avrebbe capito (as soon as he saw it, he would have understood)