Ho lasciato una banconota nel cassetto, nel caso la carta non funzioni.

Ho lasciato is the passato prossimo, a very common Italian past tense used for a completed action.

Here it means something like I left / I have left a banknote in the drawer.

It is formed with:

• ho = I have
• lasciato = past participle of lasciare (to leave)

An English speaker should note that Italian passato prossimo often covers both:

• I left
• I have left

So Ho lasciato una banconota... is the natural way to say that the speaker placed it there earlier.

Banconota means banknote or bill: a piece of paper money.

So:

• una banconota = a banknote / a bill

This is different from:

• una moneta = a coin
• i contanti = cash
• i soldi = money in general

So the sentence specifically says the speaker left a banknote, not just money in a general sense.

Because nel is a contraction of in + il.

So:

• in + il → nel

That means:

• nel cassetto = in the drawer

This kind of contraction is very common in Italian:

• in + il = nel
• in + lo = nello
• in + la = nella
• in + i = nei
• in + gli = negli
• in + le = nelle

Nel cassetto means in the drawer, so it sounds like a specific drawer that the listener can identify from context.

Italian often uses the definite article when the object is understood or already known.

Compare:

• nel cassetto = in the drawer
• in un cassetto = in a drawer

If the speaker said in un cassetto, it would sound less specific.

Nel caso means in case.

In this sentence, it introduces a precaution:

• Ho lasciato una banconota nel cassetto, nel caso la carta non funzioni.
• I left a banknote in the drawer, in case the card doesn’t work.

It gives the reason for leaving the money there: as a backup.

A learner should be careful not to confuse this with other expressions built on caso, such as:

• per caso = by chance
• in questo caso = in this case

Here la carta most naturally means the card in the sense of a payment card such as a debit or credit card.

Italian often uses la carta when the context already makes it clear what kind of card is meant. If needed, it could be more specific:

• la carta di credito = credit card
• la carta di debito = debit card

So la carta non funzioni means the card doesn’t work.

Because after nel caso in this kind of meaning, Italian commonly uses the subjunctive.

So:

• funziona = indicative
• funzioni = present subjunctive

Here, non funzioni expresses something possible or uncertain:

• in case the card doesn’t work

The sentence is really understood as something like:

• nel caso che la carta non funzioni or
• nel caso in cui la carta non funzioni

Since this is a hypothetical possibility, the subjunctive is the natural choice.

Yes, you can think of it that way.

The full version could be:

• nel caso che la carta non funzioni or, more commonly,
• nel caso in cui la carta non funzioni

In everyday Italian, the shorter version is often used:

• nel caso la carta non funzioni

So the sentence is perfectly natural even without che being stated.

Funzioni is the present subjunctive, third person singular, of funzionare (to work, to function).

Conjugation:

• che io funzioni
• che tu funzioni
• che lui/lei funzioni
• che noi funzioniamo
• che voi funzioniate
• che loro funzionino

In this sentence:

• la carta = third person singular
• so the verb is funzioni

Yes. Italian often uses funzionare for things like machines, systems, devices, and cards.

So:

• La carta non funziona = The card doesn’t work
• Il telefono non funziona = The phone isn’t working
• La stampante non funziona = The printer doesn’t work

Even though the literal sense is close to function, in normal English this is often best translated as work.

The comma separates the main statement from the extra clause explaining the reason or precaution.

Main clause:

• Ho lasciato una banconota nel cassetto

Added idea:

• nel caso la carta non funzioni

So the comma helps show:

• first, what the speaker did
• then, why they did it

You may sometimes see similar sentences without a comma, but with this kind of structure the comma is very natural.

Yes. That is a very common feature of the Italian passato prossimo.

Ho lasciato can correspond to:

• I left
• I have left

Which one sounds best in English depends on context.

For example:

• If you are simply telling someone what you did earlier, I left a banknote in the drawer may sound best.
• If the present result matters, I have left a banknote in the drawer may sound better.

Italian uses the same form here.

Yes. Some natural alternatives are:

• Ho lasciato una banconota nel cassetto, nel caso in cui la carta non funzioni.
• Ho lasciato dei contanti nel cassetto, nel caso la carta non funzioni.
• Ho lasciato una banconota nel cassetto in caso la carta non funzioni.

These all keep the same basic meaning.

A few small differences:

• nel caso in cui is a bit fuller and more explicit
• dei contanti means some cash, rather than specifically a banknote
• in caso is also common, depending on style and regional preference

It sounds neutral and natural.

Nothing in it is especially formal or especially casual. It would work well in normal spoken or written Italian.

The only slight stylistic point is that:

• nel caso in cui la carta non funzioni sounds a bit fuller and perhaps a little more careful
• nel caso la carta non funzioni sounds streamlined but still perfectly standard

So overall, the sentence is a very normal everyday Italian sentence.