Ho lasciato il computer acceso tutta la notte senza che arrivasse nemmeno un messaggio nello spam.

Because lasciare here is a normal transitive verb: it takes a direct object, il computer. In compound tenses, most transitive verbs use avere, so:

ho lasciato il computer acceso

not sono lasciato.

You use essere with some intransitive verbs and many reflexive verbs, but not with lasciare in this meaning.

This is a very common Italian pattern:

lasciare + object + adjective / past participle

It means to leave something in a certain state.

So:

ho lasciato il computer acceso = I left the computer on

Other examples:

• Ho lasciato la porta aperta = I left the door open.
• Hai lasciato la luce accesa = You left the light on.

Here acceso is not a separate action. It describes the condition of the computer after it was left.

Because acceso agrees with il computer, which is masculine singular.

• il computer acceso
• la TV accesa
• i computer accesi
• le luci accese

Also, with devices and lights, acceso usually means on / switched on, not literally lit.

This is the normal way to say all night in Italian.

• tutta la notte = all night
• tutto il giorno = all day
• tutta la settimana = all week

The adjective tutta agrees with la notte. Italian normally uses the article in expressions like this.

A learner might expect something like tutta notte, but tutta la notte is the standard everyday form.

Because what follows is a full clause with its own verb:

che arrivasse

When Italian uses a full clause after without, it often uses:

senza che + verb

So:

• senza che arrivasse nemmeno un messaggio = without even a message arriving

If you use senza by itself, it is usually followed by an infinitive or a noun:

• senza ricevere messaggi = without receiving messages
• senza messaggi = without messages

Both are possible in different sentences, but senza che is the right choice here because the sentence is built around the event a message arriving.

Because after senza che, standard Italian normally uses the subjunctive.

So:

• senza che arrivi
• senza che arrivasse
• senza che sia arrivato

depending on the tense needed.

The idea is that the clause expresses something that did not happen, or is presented as non-realized. That is why the subjunctive is natural here.

Because of the tense relationship with ho lasciato.

Main clause:

• Ho lasciato il computer acceso tutta la notte → past event

Subordinate clause:

• senza che arrivasse... → something that did not happen during that same past time span

So Italian uses the imperfect subjunctive: arrivasse.

Very roughly:

• arrivi would fit a present-time frame
• arrivasse fits a past frame like this one
• sia arrivato would suggest a completed action seen from a different angle, and sounds less natural here

So arrivasse is the most natural match.

The subject is un messaggio.

Italian often places the subject after the verb, especially when introducing something that did or did not happen:

senza che arrivasse nemmeno un messaggio

Literally, the structure is close to: without there arriving even one message

This word order sounds natural in Italian. You could move the subject earlier, but it would usually sound less smooth.

Nemmeno means even in a negative context, so here it is basically not even.

So:

nemmeno un messaggio = not even one message

It adds emphasis: the speaker is saying that the result was zero messages, and in fact not even one ended up there.

Common near-synonyms are:

• neanche
• neppure

In many contexts, they are interchangeable.

Because Italian often uses the singular after words like nemmeno to mean not even one.

So:

• nemmeno un messaggio = not even one message

This is more emphatic than a plain plural idea like no messages. It highlights the total absence of even a single item.

Here nello spam means something like in the spam folder or in spam.

Nello is the contraction of:

• in + lo = nello

And spam is treated as a masculine noun here:

• lo spam
• nello spam

That article form is also due to the initial sp-, which takes lo in standard Italian:

• lo sport
• lo spam

A more explicit version would be:

• nella cartella spam
• nella posta indesiderata

But nello spam is a very natural shorthand in everyday tech-related Italian.