Learn Hardcore Italian: Se perdessi il passaporto, la vacanza diventerebbe un problema. - If I lost my passport, the holiday would become a problem.

Because this is a hypothetical or unreal “if” clause (the protasis). In Italian, when you talk about something contrary to fact or unlikely, you use se + imperfect subjunctive (congiuntivo imperfetto). Here perdessi corresponds to “if I lost” in English, emphasizing that the loss is just an imagined scenario.

Why is diventerebbe in the conditional tense?

The main clause (apodosis) of a hypothetical sentence uses the present conditional (condizionale presente) to express what would happen if the condition were met. diventerebbe (“would become”) matches the imperfect subjunctive perdessi to complete the so-called second conditional structure.

How do you form the imperfect subjunctive of a regular –ere verb like perdere?

For regular –ere verbs, remove the infinitive ending -ere and add the subjunctive endings:
io -essi, tu -essi, lui/lei -esse, noi -essimo, voi -este, loro -essero.
So: io perdessi, tu perdessi, lui/lei perdesse, noi perdessimo, voi perdeste, loro perdessero.

When would you use the indicative after se instead of the subjunctive?

Use the indicative in real or likely conditions (first conditional). For example:
Se perdi il passaporto, la vacanza diventerà un problema.
Here you’re talking about a real possibility, so you pair the present indicative in the se clause with the future or imperative in the main clause.

Is the comma before la vacanza mandatory?

Not strictly mandatory, but it’s standard practice to separate the subordinate se clause from the main clause with a comma when the se clause comes first. It improves readability:
Se perdessi il passaporto, la vacanza diventerebbe un problema.

Why is there a definite article in la vacanza but an indefinite article in un problema?

la vacanza refers to “the vacation” you already have in mind (specific context), so it takes the definite article. un problema introduces “a problem” for the first time, so it takes the indefinite article.

What’s the difference between la vacanza and il viaggio?

la vacanza emphasizes a period of leisure or time off (vacation), while il viaggio focuses on the act or journey itself (trip). You can have a vacation (vacanza) that includes several trips (viaggi), or vice versa.

Could you rephrase the sentence using different tenses for a past unreal condition?

Yes. To speak about a hypothetical past event, you’d use the past perfect subjunctive in the se clause and the past conditional in the main clause:
Se avessi perso il passaporto, la vacanza sarebbe diventata un problema.