Il fornaio mi tiene da parte dodici panini, mentre la fruttivendola pesa tredici mele rosse.

Because Italian nouns have grammatical gender, and the definite article must agree with the noun:

• il fornaio = the baker
  
  • fornaio is masculine singular, so it takes il
• la fruttivendola = the greengrocer / fruit seller
  
  • fruttivendola is feminine singular, so it takes la

If the seller were male, you would usually say il fruttivendolo.

Mi means to me or for me.

In this sentence:

• Il fornaio mi tiene da parte dodici panini

the idea is that the baker is keeping the rolls aside for me.

This mi is an unstressed object pronoun, and in Italian it normally goes before the conjugated verb:

• mi tiene
• mi dice
• mi porta

English learners often expect something like per me, but with many verbs Italian prefers a pronoun like mi.

Because object pronouns like mi, ti, gli, le, ci, vi usually come before a normal finite verb in Italian.

So Italian says:

• mi tiene da parte
• mi parla
• mi scrive

not tiene mi.

You do get pronouns attached to the end with infinitives, gerunds, and imperatives, for example:

• tenermi
• tenendomi
• dimmi

But in this sentence, tiene is a normal present-tense form, so mi comes before it.

Tenere da parte is an expression meaning to keep aside, to set aside, or to reserve.

So:

• Il fornaio mi tiene da parte dodici panini

means the baker keeps twelve rolls aside for me.

It is not just the literal meaning of tenere = to hold. The full expression tenere da parte has its own idiomatic meaning.

A close alternative in Italian could be:

• mi mette da parte dodici panini

which also means sets aside twelve rolls for me.

Because dodici panini is simply the direct object: twelve rolls.

After numbers, Italian usually uses:

• number + noun

So:

• dodici panini
• tredici mele
• due libri

You would not normally use dei here unless you were expressing a partitive idea in a different context. In this sentence, the speaker just means a specific quantity of items.

In Italian, panini is plural.

• panino = one roll / sandwich
• panini = rolls / sandwiches

This is a very common point of confusion for English speakers because in English panini is often used as if it were singular. But in Italian, the singular is panino.

So:

• un panino
• due panini

Mentre usually means while, and it can also give a sense of contrast, like whereas.

Here it links two actions happening at the same time, or two parallel actions:

• the baker sets aside twelve rolls for me
• the greengrocer weighs thirteen red apples

So mentre is more expressive than just e because it suggests while this is happening, that is happening too.

Pesa is from pesare, meaning to weigh.

In this context, la fruttivendola pesa tredici mele rosse means the greengrocer is weighing thirteen red apples, or weighs out thirteen red apples.

Italian present tense often covers both English meanings:

• weighs
• is weighing

So the exact English translation depends on context, but the Italian form is the same.

Because the adjective must agree with the noun in gender and number.

• mela is feminine singular
• mele is feminine plural

So the adjective rosso must become feminine plural too:

• mela rossa = red apple
• mele rosse = red apples

This agreement is one of the most important features of Italian grammar.

Because in Italian, descriptive adjectives often come after the noun.

So:

• mele rosse = red apples
• panini caldi = hot rolls
• casa grande = big house

Sometimes adjectives can come before the noun, but the usual neutral position for a color adjective like rosse is after the noun.

Because when a noun is directly quantified by a number, Italian normally does not add an article.

So you say:

• dodici panini
• tredici mele rosse
• tre libri

not normally i dodici panini unless you specifically mean the twelve rolls in a context where they are already identified.

In this sentence, the focus is on the quantity, so no article is needed.

It can mean either, depending on context.

• Il fornaio mi tiene da parte dodici panini can mean:
  • The baker keeps twelve rolls aside for me
  • The baker is keeping twelve rolls aside for me
• La fruttivendola pesa tredici mele rosse can mean:
  • The greengrocer weighs thirteen red apples
  • The greengrocer is weighing thirteen red apples

Italian often uses the simple present where English might prefer either simple present or is/are + -ing.

Yes. They are related words, but they refer to different genders:

• fruttivendolo = male greengrocer / fruit seller
• fruttivendola = female greengrocer / fruit seller

This is very common in Italian with profession nouns, though usage can vary depending on the noun. In this sentence, la fruttivendola clearly refers to a woman.