Appena corresponds to the English conjunction as soon as. Appena can be used in a context which describes a reality and in a context which describes a hypothetical situation. The general rule for all romance language is to be applied. In the context of a real situation we use the indicativo, in the case of a hypothetical situation the congiuntivo. The only conjunction where this rule is not applied is, as we have seen already, quando.
real situation
As soon as I got the money, I bought the car. hypothetical situation
As soon as I get the money, I will buy a car.
examples
with indicativo
La vidi non appena entrai nella sala.
As soon as I entered the room, I saw her.
Non appena la porta della stanza si fu richiusa alle sue spalle, ebbe un tremito.
As soon as the door was closed behind him a chill ran down his back.
Non appena smetterà di piovere, andremo alla spiaggia.
As soon as it had stopped raining, we went to the beach.
Non appena avrai inviato il tuo ordine, riceverai un'e-mail di conferma.
Once you have submitted your order, you will receive a confirmation email.
Appena disse queste parole io mi misi a piangere perchè era da tanto che desideravo un cane.
No sooner had he said that, I started crying because since so long I wished a dog.
Pur essendo il mondo intero pieno di sciocchi, non c'è nessuno che si creda tale o appena abbia* il sospetto di esserlo.
Although the world is full of idiots there is no one who believes to be one or who supposes at least to be one.
* the congiuntivo in this case doesn't point to the hypothetical character but depends on the verb credere which always (in Italian but not in Spanish or French!) requires the congiuntivo. Appena in this sentence is an adverb, not a conjunction.
with congiuntivo
Io lo comprerò appena sarà in vendita!
I will buy it as soon as it is commercially available.
Il compratore deve ritirare la merce dalla nave non appena sia stata messa a sua disposizione.
The buyer is obliged to take the merchandise from the ship as soon as it is available to him.
Le nuove regole impongono ai professionisti di segnalare le operazioni sospette, non appena siano venuti a conoscenza di elementi che facciano sospettare la provenienza illecita del denaro.
The new rule oblige the tradesmen to notify conspicuous transactions once they get informed about circumstances that suggests an illegal origin of the money.
Per quanto riguarda invece le professioni non regolamentate, si può iniziare a praticare non appena si sia ottenuto un impiego, ma occorre osservare tutte le norme applicabili alla professione nel paese ospitante, norme che possono essere diverse da quelle del paese d'origine.
Concerning the non regulated professions one can start as soon as one find a job but the rules to apply of the host country which may be different than the rules of the country of origin are to be respected.
Appena can be an adverb as well as a conjunction.
appena as an adverb
Ho appena smesso di fumare.
I have just now stopped smoking.
Violenza carnale su un bimbo di appena 10 anni.
Physical violence against a ten-year-old child.
appena as a conjunction
Appena ci siamo resi conto del pericolo, siamo subito scappati dalla spiaggia, siamo saliti in macchina per allontanarci dalle fiamme.
As soons as we became aware of the danger we immediately escaped from the beach, got in our car to get away from the fire.
