Prima di uscire, metto dodici biscotti in una scatola e lascio tredici euro sul tavolo.

Italian often drops subject pronouns because the verb ending already shows who is doing the action.

• metto = I put
• lascio = I leave

So io metto is possible, but io is usually only added for emphasis, contrast, or clarity.

For example:

• Metto dodici biscotti... = I put twelve biscuits...
• Io metto dodici biscotti, tu ne metti dieci. = I put twelve biscuits, you put ten.

Because Italian uses prima di + infinitive to mean before doing something.

So:

• prima di uscire = before going out / before leaving

This is a very common pattern:

• prima di mangiare = before eating
• prima di dormire = before sleeping
• prima di partire = before leaving

You cannot normally say prima uscire in this meaning.

Because uscire depends on prima di, which is followed by the infinitive.

Meanwhile, metto and lascio are the main actions of the sentence, so they are conjugated.

• prima di uscire = before going out
• metto = I put
• lascio = I leave

So the structure is:

Before going out, I put twelve biscuits in a box and leave thirteen euros on the table.

They are in the present indicative.

• metto = present tense of mettere
• lascio = present tense of lasciare

In Italian, the present tense can describe:

• something happening now
• a habitual action
• a general routine
• sometimes even a near-future action, depending on context

Here it most naturally sounds like a routine or simple statement of action.

They are similar in that both involve placing something somewhere, but they are not identical.

• mettere = to put / place
• lasciare = to leave

So:

• metto dodici biscotti in una scatola = I put twelve biscuits into a box
• lascio tredici euro sul tavolo = I leave thirteen euros on the table

Mettere focuses on the act of placing something. Lasciare often suggests that something remains there afterward.

Because the prepositions match the physical relationship:

• in una scatola = in a box / inside a box
• sul tavolo = on the table

Also, sul is a contraction:

• su + il = sul

So:

• sul tavolo literally = on the table

This is very common in Italian:

• sulla sedia = on the chair
• nel cassetto = in the drawer
• sotto il letto = under the bed

Because they are doing different jobs.

• una scatola uses the indefinite article: a box
• sul tavolo uses the definite article inside the contraction: on the table

So the sentence is talking about:

• a box that is not specifically identified beforehand
• the table, which is treated as a known or contextually understood object

Italian uses definite articles more often than English does, especially with places and everyday objects.

No. Dodici and tredici are invariable cardinal numbers.

They stay the same with masculine and feminine nouns:

• dodici biscotti
• dodici mele
• tredici euro
• tredici sedie

The noun changes as needed, but the number does not.

Because the noun must be plural after dodici.

• un biscotto = one biscuit
• due biscotti = two biscuits
• dodici biscotti = twelve biscuits

The singular form is biscotto. The plural form is biscotti.

In standard Italian, euro is usually invariable after numbers.

So you say:

• un euro
• due euro
• tredici euro

It does not normally change form in the plural.

This is different from many regular Italian nouns, but it is the standard usage.

Yes, that is the normal interpretation.

In a structure like prima di + infinitive, the subject is usually understood to be the same as the subject of the main verb, unless context clearly suggests otherwise.

So here the natural reading is:

Before I go out, I put twelve biscuits in a box and leave thirteen euros on the table.

If the subject were different, Italian would usually use a different structure, often with prima che:

• Prima che lui esca, metto i biscotti... = Before he goes out, I put the biscuits...

It is not absolutely required, but it is very natural.

Prima di uscire is an introductory phrase, and Italian often uses a comma after an introductory element, especially when it comes at the beginning of the sentence.

So both are possible:

• Prima di uscire, metto dodici biscotti...
• Prima di uscire metto dodici biscotti...

The version with the comma is a little clearer and more polished in writing.