Se il salvadanaio fosse pieno, comprerei un nuovo libro.

Fosse is the imperfetto congiuntivo of essere, used after se to express an unreal or hypothetical situation.

• Era or è are indicative forms and would suggest a real past or present fact.
• In this sentence we imagine an unlikely or purely hypothetical condition (“If the piggy bank were full…”), so Italian requires the subjunctive.

Italian has three main “if”-clauses:

1. Real condition (indicative + future):
   Se il salvadanaio è pieno, comprerò un nuovo libro.
2. Hypothetical present/future (imperfetto congiuntivo + condizionale presente):
   Se il salvadanaio fosse pieno, comprerei un nuovo libro. ← our sentence
3. Impossible past (trapassato congiuntivo + condizionale passato):
   Se il salvadanaio fosse stato pieno, avrei comprato un nuovo libro.
   Here we talk about a present/future hypothesis that isn’t really happening, so we use the imperfect subjunctive fosse, not the present sia.

This is a second-type conditional (also called “unreal present” or “hypothetical present/future”):

• Protasis (if-clause): se + imperfetto congiuntivo
• Apodosis (result clause): condizionale presente

It expresses what you would do if something unlikely in the present were true.

Essere (imperfetto congiuntivo):

• io fossi
• tu fossi
• lui/lei fosse
• noi fossimo
• voi foste
• loro fossero

Comprare (condizionale presente):

• io comprerei
• tu compreresti
• lui/lei comprerebbe
• noi compreremmo
• voi comprereste
• loro comprerebbero

In our sentence we use fosse (3rd sg.) and comprerei (1st sg.) to match the subjects.

Both are correct, but the nuance changes slightly:

• Un nuovo libro (adj. before noun) often implies “another/brand-new book.”
• Un libro nuovo (adj. after noun) focuses on the book’s new condition, perhaps contrasting with an old one.
  In everyday speech they’re interchangeable in most contexts.

Salvadanaio literally comes from salvare (to save) + an old form of danaro (money). It means “piggy bank” or coin bank.
Synonyms or variants:

• porcellino salvadanaio (if shaped like a pig)
• cassettina per monetine (more formal, “small coin box”)