Substantive: Numerus

English makes most plurals with -s. Italian changes the final vowel instead, and the new vowel depends on the old one. Adding an -s (*pizzas, *gelatos) is the most common slip English speakers make.

That is why panini, spaghetti and zucchini are already plural in Italian: one sandwich is un panino.

Keeping the hard sound: -co, -go, -ca, -ga

Adding -i or -e after C or G would soften the sound (remember ci = "chee"). To keep it hard, Italian adds an H:

The -co nouns split, so learn the plural with the word. A rough guide: stressed on the next-to-last syllable keeps the hard sound (PAR-co → parchi), stressed earlier softens (ME-di-co → medici), with amico → amici as the famous exception.

Nouns that do not change

• Nouns ending in a stressed vowel: la città → le città, il caffè → i caffè, l'università → le università.
• Shortened words: la foto → le foto, il cinema → i cinema, la moto → le moto.
• Foreign words: il bar → i bar, il film → i film. Italian does not add the English -s, so it is due film, not *due films.
• Nouns ending in a consonant or in -i: la crisi → le crisi.

In these the number shows on the article and on any adjective: un caffè lungo, due caffè lunghi.

A few irregulars

l'uomo → gli uomini - man, men; l'uovo → le uova - egg(s); la mano → le mani - hand(s). They are frequent enough to learn now.