Generally to make the plural of Italian nouns you only have to change the last vowel. So:
masculine ending in -o (sing) = -i (pl)
feminine ending in -a (sing) = -e (pl)
For example:
masculine: amico – amici (friend/friends), libro – libri (book/books), tavolo – tavoli (table/tables)
feminine: penna – penne (pen/pens), sedia – sedie (chair/chairs), matita – matite (pencil/pencils)
If the noun ends in -e, then the plural is made with an -i
ristorante – ristoranti (restaurant/restaurants), fiore – fiori (flower/flowers)
There are some exceptions though:
– words ending in -tà and -tù stay the same in the plural: città – città (city/cities)
– foreign words ending with a consonant stay the same: bar – bar (bar/bars)
– some of the masculine nouns referring to the body become feminine in the plural ending in -a:
braccio – braccia (arm/arms), dito – dita (finger/fingers), ginocchio – ginocchia (knee/knees)
When the plural refers to two or more nouns of different genders, the masculine plural is used:
5 ragazze (fem) + 1 ragazzo (masc) = 6 ragazzi (masc)