Marta stampa il curriculum e lo mette nella cartellina.

Because curriculum is treated as a masculine singular noun in Italian, so it takes il.

• il curriculum = the résumé / the CV
• pronoun that replaces it: lo = it

Even though the English meaning may not feel especially masculine, grammatical gender in Italian depends on the Italian noun, not on English.

Here stampa is the 3rd person singular present tense of stampare.

So:

• io stampo = I print
• tu stampi = you print
• lui/lei stampa = he/she prints

In this sentence, the subject is Marta, so stampa means prints.

Yes, Italian often omits subject pronouns, because the verb ending usually shows who is doing the action.

For example:

• Stampa il curriculum e lo mette nella cartellina.

could work in the right context if it is already clear that we are talking about Marta.

But here Marta is included to make the subject explicit.

Lo is a direct object pronoun meaning it. It refers back to il curriculum.

So:

• Marta stampa il curriculum = Marta prints the résumé
• e lo mette nella cartellina = and puts it in the folder

Italian usually avoids repeating the noun when a pronoun can replace it, just like English often does.

Because lo must agree with the noun it replaces: il curriculum, which is masculine singular.

Italian direct object pronouns include:

• lo = him / it (masculine singular)
• la = her / it (feminine singular)
• li = them (masculine plural)
• le = them (feminine plural)

Since curriculum is masculine singular, the correct pronoun is lo.

In Italian, direct object pronouns usually come before a conjugated verb.

So Italian says:

• lo mette = puts it

not:

• mette lo ❌

This is very normal in Italian word order.
Compare:

• La vedo = I see her
• Lo prendo = I take it
• Li compro = I buy them

Mette is the 3rd person singular present tense of mettere = to put.

Conjugation:

• io metto
• tu metti
• lui/lei mette
• noi mettiamo
• voi mettete
• loro mettono

So Marta mette means Marta puts.

Nella is a contraction of:

• in + la = nella

It means in the.

So:

• nella cartellina = in the folder

This kind of contraction is very common in Italian:

• nel = in + il
• nello = in + lo
• nella = in + la
• nei / negli / nelle = plural forms

Cartellina usually means folder, file folder, or document folder.

It comes from cartella, and the ending -ina is a diminutive, which often gives the idea of something smaller or sometimes just a common everyday form.

So cartellina often suggests a folder for papers or documents.

It is feminine singular, which is why we get:

• la cartellina
• nella cartellina

Because the sentence is talking about a specific folder, so Italian uses the definite article:

• nella cartellina = in the folder

If it were nonspecific, you might say:

• in una cartellina = in a folder

Italian uses articles more often than English, so where English might sometimes just say in folder in a note-like style, standard Italian normally wants an article.

The word curriculum is a borrowing from Latin, and in modern Italian it is commonly treated as invariable in everyday use.

So you often see:

• il curriculum
• i curriculum

In real usage, many people also say curriculum vitae for the singular, and context makes the meaning clear.

For this sentence, though, it is singular: il curriculum.

Yes, it could say:

• Marta stampa il curriculum e mette il curriculum nella cartellina.

But that sounds repetitive. Italian, like English, normally prefers the pronoun once the object is already clear:

• Marta stampa il curriculum e lo mette nella cartellina.

That is the more natural version.

Yes. It is in the simple present:

• stampa
• mette

Italian often uses the present tense to describe:

• routines
• instructions
• actions in a narrative
• a sequence of steps

So the sentence can describe what Marta does as part of a process, even if in English you might sometimes translate it with a slightly different feel depending on context.