Italians nouns and adjectives follow this simple rule to create their plural: unstressed feminine -a become -e, unstressed -o, -e and masculine -a become -i, while the rest of the words remain invariable. Can you find the exceptions to this rule?
Source: Wikipedia
The words ending in -ca and -ga always turn into -che and -ghe, but those in -co and -go only become -chi and -ghi directly after an unstressed vowel. Although the suffix -logo remains -logi if it refers to a person.
Some words change genders when they pluralize, including every -o turning into -a.
A lot of these irregular nouns also have a regular alternative, sometimes with a different meaning.