Plurals

Italian never adds -s. It changes the final vowel instead — the one habit English and Spanish speakers must unlearn.

The core system

singular ends in	plural becomes	example
-o (m)	-i	libro → libri
-a (f)	-e	casa → case
-e (m or f)	-i	cane → cani, notte → notti
-a (m — the -ma words)	-i	problema → problemi, turista → turisti

So -i is the general plural; -e is only for feminine -a nouns.

Whether the hard sound survives depends on stress — with famous exceptions:

type	rule	examples
-ca / -ga (f)	always keep the sound: -che / -ghe	amica → amiche, riga → righe
-co / -go, stress on the syllable before	keep it: -chi / -ghi	fuoco → fuochi, albergo → alberghi
-co / -go, stress earlier	soften: -ci / -gi	medico → medici, psicologo → psicologi
the exception everyone meets first		amico → amici, greco → greci, nemico → nemici

Note the asymmetry: amica → amiche (hard) but amico → amici (soft).

-io with unstressed i: one i in the plural — figlio → figli, bacio → baci. Stressed í doubles: zio → zii.
-cia / -gia: keep the i after a vowel (camicia → camicie, valigia → valigie), drop it after a consonant (arancia → arance, pioggia → piogge).

No change at all for:

type	examples
stressed final vowel	la città → le città, il caffè → i caffè
loanwords ending in a consonant	il bar → i bar, il film → i film, lo sport → gli sport
one-syllable and abbreviated words	il re → i re, la foto → le foto, l'auto → le auto

The article carries the plural: il film → i film.

uomo → uomini (man), dio → dei (god).
The gender-switchers: il braccio → le braccia, l'uovo → le uova — see Nouns & gender.

Spanish/French add -s (casas, maisons); Italian changes the vowel (case). In speech this makes Italian plurals far more audible than French ones.
Because there is no -s, you always hear number in the article + ending pair: la casa / le case, il cane / i cani.