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Plural compounds: change the first noun, leave the second alone

Italian often builds new words by putting two nouns side by side: nome utente (username), parola chiave (keyword), stazione radio (radio station), anno luce (light year). The first noun is the real thing you are talking about. The second noun simply says what kind of thing it is. When you make these combinations plural, only the first noun changes: nomi utente, parole chiave, stazioni radio, anni luce.

English speakers tend to get this backwards. In English the plural goes at the end (usernames, keywords, light years), so the instinct is to change the second Italian word, or to change both. Neither is right for this group of words.

Word list
sito internetmA2 website i siti internet sito webmA2 website i siti web connessione InternetfB1 internet connection le connessioni Internet indirizzo e-mailmB1 e-mail address gli indirizzi e-mail nome utentemB1 username i nomi utente parco divertimentimB1 amusement park i parchi divertimenti piano terramB1 ground floor i piani terra (not piani terre) ritiro bagaglimB1 baggage claim i ritiri bagagli (bagagli already plural, unchanged) stazione radiofB1 radio station le stazioni radio anno lucemB2 light year gli anni luce assicurazione malattiafB2 health insurance le assicurazioni malattia banca datifB2 database le banche dati (dati already plural, unchanged) cibo spazzaturamB2 junk food i cibi spazzatura nave cargofB2 cargo ship le navi cargo parco giochimB2 playground i parchi giochi parola chiavefB2 keyword le parole chiave (parole chiavi also occurs) borsa valorifC1 stock exchange le borse valori campo profughimC1 refugee camp i campi profughi cerniera lampofC1 zipper le cerniere lampo comunicato stampamC1 press release i comunicati stampa conferenza stampafC1 press conference le conferenze stampa forza lavorofC1 workforce le forze lavoro (mostly singular, a collective noun) pesce spadamC1 swordfish i pesci spada cavallo vaporemC2 horsepower i cavalli vapore pesce gattomC2 catfish i pesci gatto stato fantocciomC2 puppet state gli stati fantoccio
How it works

Think of the first noun as the head and the second as a label. In stazione radio, a stazione is a station, and radio tells you which kind. Because the head is what you are counting, the head takes the plural ending. The label stays frozen.

English does the opposite. English puts the label first and the head last (radio station), and the plural -s goes on the head at the end. Italian puts the head first, so the plural ending lands at the front.

The head also decides the gender. That is why it is la banca dati (feminine, like banca) and il pesce spada (masculine, like pesce). Articles and adjectives agree with the first noun too: gli anni luce, le conferenze stampa.

Le conferenze stampa si tengono in due sale diverse.
(The press conferences take place in two different rooms.)

Rule of thumb: ask yourself which word is the thing and which word describes it. Pluralize the thing.

The words
Online and everyday tech

You will meet these on any website or app: nome utente, indirizzo e-mail, parola chiave, banca dati, plus the older stazione radio.

Questo sito non accetta nomi utente con spazi.
(This site doesn't accept usernames with spaces.)
Ho due indirizzi e-mail: uno per il lavoro e uno personale.
(I have two e-mail addresses: one for work and one personal.)
Inserisci tre parole chiave.
(Enter three keywords.)
In macchina ascolto sempre le stesse stazioni radio.
(In the car I always listen to the same radio stations.)
L'università ha accesso a molte banche dati scientifiche.
(The university has access to many scientific databases.)
Places

With parco divertimenti, parco giochi and campo profughi, the second noun is already plural in the singular form. You still only touch the first noun.

In estate i parchi divertimenti sono pieni di turisti.
(In summer amusement parks are full of tourists.)
Nel nostro quartiere ci sono due parchi giochi.
(There are two playgrounds in our neighborhood.)
Molte famiglie vivono ancora nei campi profughi vicino al confine.
(Many families still live in the refugee camps near the border.)
The press

Besides conferenza stampa, you will often see comunicato stampa. In both, stampa never changes.

L'azienda ha pubblicato due comunicati stampa in una settimana.
(The company published two press releases in one week.)
Animals

Fish named after something else follow the same pattern: pesce spada and pesce gatto.

Al mercato oggi c'erano pesci spada enormi e qualche pesce gatto.
(At the market today there were huge swordfish and a few catfish.)
Measures, objects and other labels

anno luce, cavallo vapore, nave cargo, cerniera lampo, cibo spazzatura and stato fantoccio all work the same way.

Quella stella è a migliaia di anni luce da noi.
(That star is thousands of light years away from us.)
Il motore ha centocinquanta cavalli vapore.
(The engine has a hundred and fifty horsepower.)
Nel porto aspettano cinque navi cargo e lo zaino ha tre cerniere lampo rotte.
(Five cargo ships are waiting in the port, and the backpack has three broken zippers.)
I cibi spazzatura costano poco, ma gli stati fantoccio della storia costavano caro ai loro cittadini.
(Junk foods are cheap, but history's puppet states cost their citizens dearly.)
Watch out

The typical mistake is to pluralize the end: nome utenti or stazione radii. Pluralizing both is also wrong. The only exception you will see in real use is parole chiavi, which does occur, but parole chiave is the standard form and the safer choice.

Some second nouns already look plural, like dati in banca dati or giochi in parco giochi. Do not try to change them into anything. Just pluralize the head: banche dati, parchi giochi.

This rule covers separate-word combinations where the second noun is a label. It is not a rule for every compound. Words written as one, such as cassaforte (safe) and pescecane (shark), have their own plurals: casseforti, pescecani. When the second word works more like an adjective, it may agree as well: people say lingue madri.

Summary

In two-word nouns like nome utente, parola chiave or conferenza stampa, the first noun is the thing and the second is only a label. Make the first noun plural, keep its gender for articles and adjectives, and leave the second noun exactly as it is: nomi utente, parole chiave, conferenze stampa.