In short

Some nouns refuse the vowel swap you learned in step 2: la città and il film never change, lo zio gives gli zii but lo studio gives gli studi, and il braccio turns feminine in the plural - le braccia. The same lesson gives you the suffix kit Italian uses to build new words from old ones: un tavolino is a small tavolo, un tempaccio is horrible weather, rileggere is to read again.

The rule

Part 1 - Nouns that don’t move

Step 1 - If the last vowel is stressed and written with an accent, the noun is invariable. la città -> le città, il caffè -> i caffè, la virtù -> le virtù, il lunedì -> i lunedì. There is nowhere to put a plural ending: the stress is already sitting on the final vowel. Le cittàs and le citte are both impossible.

Step 2 - If the word came from another language and ends in a consonant, it is invariable too. il film -> i film, il bar -> i bar, il computer -> i computer, lo sport -> gli sport. Italian does not import the English -s: i films is a mistake, not an option.

Step 3 - Feminine nouns in -i are invariable. Nearly all are Greek-built abstract words: la crisi -> le crisi, l’analisi -> le analisi, la tesi -> le tesi, l’ipotesi -> le ipotesi.

Step 4 - Also invariable: one-syllable nouns and shortened words. il re -> i re, lo sci -> gli sci; la foto -> le foto, la moto -> le moto, la bici -> le bici, l’auto -> le auto, il cinema -> i cinema. The short forms keep the gender of the long word they came from (la fotografia -> la foto, il cinematografo -> il cinema), which is why foto is feminine despite the -o.

In every one of these, the article and the adjective carry the number by themselves: la crisi economica -> le crisi economiche.

Part 2 - Nouns in -io: one i or two?

A noun in -io loses its -o in the plural, exactly like libro -> libri. The question is what happens to the i that is left. Listen for the stress.

• Stress on the i -> the plural has -ii. zì-o -> gli zii, addì-o -> gli addii, pendì-o -> i pendii, mormorì-o -> i mormorii.
• Stress before the i (the normal case) -> a single -i. stù-dio -> gli studi, fì-glio -> i figli, viàg-gio -> i viaggi, esèm-pio -> gli esempi, òc-chio -> gli occhi.

The unstressed group is far bigger, so single -i is the default and the -ii words are a short list to memorise. The pair to hang it on is gli zii (uncles) against gli studi (studies, offices).

Part 3 - Masculine singular, feminine plural

A small but very frequent class is masculine in the singular and feminine in the plural, ending in -a. These are Latin neuters that survived: il braccio -> le braccia, and the plural takes le and feminine adjectives - le braccia sono stanche.

They cluster in two areas: parts of the body (braccia, dita, ginocchia, labbra, ciglia, ossa) and measures and quantities (paia, centinaia, migliaia, miglia), plus a few household words (uova, lenzuola).

Several of them have two plurals with different meanings: the feminine -a plural means the natural set, the regular masculine -i plural means separate items or a figurative sense. Le braccia are your arms; i bracci are the arms of a river or of a lamp.

Part 4 - Alterati: changing the size, not the word

Italian rarely says piccolo or grande when a suffix will do. Drop the final vowel of the noun and add the suffix; the gender and the number stay as they were: il gatto -> il gattino, i gatti -> i gattini, la casa -> la casetta.

• -ino, -etto, -ello, -uccio make it small (and often affectionate): il tavolino, il libretto, l’asinello, la boccuccia.
• -one makes it big: il librone, il ragazzone. Careful: -one is masculine even on a feminine noun. La porta -> il portone, la palla -> il pallone, la scarpa -> lo scarpone.
• -accio makes it bad: il tempaccio (foul weather), la parolaccia (swear word), la giornataccia (awful day). -astro means shoddy: il poetastro.

The trap: not every -ino or -one is an alterato. Many words simply end that way and mean something else entirely. Un tacchino is a turkey, not a small heel (il tacco); un burrone is a ravine, not a lot of butter (il burro). These are called falsi alterati and they are ordinary vocabulary - see the table below.

Second trap: the suffixes are not free. Un momentino and una casetta exist because Italians say them; an invented casona or problemino may be understood but sound wrong. Produce the ones you have heard; when in doubt, say piccolo/grande + noun, which is never wrong.

Part 5 - Building words: prefixes and suffixes

The same machine builds whole new words, and recognising it doubles your vocabulary for free.

• ri- = again: leggere -> rileggere, fare -> rifare, vedere -> rivedere, chiamare -> richiamare (to call back).
• s- = undo / the opposite: gonfiare -> sgonfiare (to deflate), caricare -> scaricare (to unload, to download), fortunato -> sfortunato, comodo -> scomodo, cortese -> scortese.
• in- = not (on adjectives), and it assimilates to the letter after it: utile -> inutile, but possibile -> impossibile (before p, b, m), legale -> illegale (before l), regolare -> irregolare (before r). dis- does a similar job: onesto -> disonesto, ordine -> il disordine.
• -zione turns a verb into the feminine noun of the action: preparare -> la preparazione, informare -> l’informazione. -mento does the same with a masculine noun: cambiare -> il cambiamento, sentire -> il sentimento, pagare -> il pagamento.
• -oso turns a noun into an adjective meaning full of: il pericolo -> pericoloso, la noia -> noioso, il coraggio -> coraggioso, il silenzio -> silenzioso.

Forms & patterns

Nouns that never change - the article does all the work:

Nouns in -io - the stress decides:

Masculine singular -> feminine plural:

Two plurals, two meanings:

The alteration suffixes:

Falsi alterati - ordinary words that only look altered:

Word formation at a glance:

Examples

Common mistakes

• Ho visto tre films. -> Ho visto tre film. Foreign nouns are invariable in Italian and never take the English -s. The number is on the article and the number word.
• gli studii -> gli studi The i of studio is unstressed, so only one i survives. Two i appear only when the i is stressed: gli zii, gli addii.
• Mi fanno male i bracci. -> Mi fanno male le braccia. For the body, the plural is feminine in -a. I bracci exists, but it means the arms of a river or of a lamp.
• Ho comprato sei uove. -> Ho comprato sei uova. L’uovo is masculine in the singular and feminine in the plural, and its plural is le uova - not a regular -e.
• La portone è chiusa. -> Il portone è chiuso. The suffix -one makes the word masculine even though la porta is feminine, so the article and the adjective both flip.

Tips

• Invariable does not mean invisible. If the noun can’t change, everything around it must: la crisi economica -> le crisi economiche, il film francese -> i film francesi. And never “fix” an invariable noun by dropping its accent - città without the accent is not a word.
• For -io, say the word out loud and find the stress. zì-o -> zii; stù-dio -> studi. Single -i is the default; the -ii plurals are a handful of words you can learn as vocabulary.
• The feminine -a plurals are the body and the counting words. Ten items cover almost all of it: braccia, dita, ginocchia, labbra, ciglia, ossa, uova, lenzuola, paia, centinaia. Learn each with le, because the article is what makes them sound right.
• Alterati: recognise freely, produce carefully. Understanding un caffettino or un problemaccio costs nothing; inventing your own risks a tacchino. If you have not heard the form, piccolo and grande are always safe.