Sostantivi: genere e numero/2

In Italian there are some nouns that behave in a specific way.

Some feminine nouns end in –o or in –i and their plural is invariable:
la radio/le radio - the radio/the radios
la moto/le moto - the motorbike/the motorbikes
but la mano/le mani - the hand/the hands
la tesi/le tesi - the thesis/the theses
la crisi/le crisi - the crisis/the crises
la bici/le bici - the bike/the bikes

Some masculine nouns end in –a and their plural is in –i:
il problema/i problemi - the problem/the problems
il poeta/i poeti - the poet/the poets

Nouns ending in –tà and –tù are feminine and their plural is invariable:
la città/le città - the town/the towns
l’università/le università - the università/the universities
la virtù/le virtù - the virtue/the virtues

Nouns ending with a consonant are generally masculine and their plural is invariable:
il computer/i computer - the computer/the computers
l’autobus/gli autobus - the bus/the buses
il film/i film - the film/the films

Nouns ending in –ista can be both masculine and feminine and their plural varies accordingly:
l’artista/gli artisti-le artiste - the artist/the artists
il-la farmacista/i farmacisti-le farmaciste - the pharmacist/the pharmacists

Nouns ending in –co and –go can form their plural in –chi and –ghi or in –ci and –gi:
il fiasco/i fiaschi - the flask/the flasks
l’albergo/gli alberghi - the hotel/the hotels
l’amico/gli amici - the friend/the friends
lo psicologo/gli psicologi - the psychologist/the psychologists

Feminine nouns ending in –ca and –ga form their plural in –che and –ghe:
l’amica/le amiche - the friend/the friends
la collega/le colleghe - the colleague/the colleagues

Some masculine nouns (mostly indicating the parts of the body) have a feminine plural:
il dito/le dita - the finger/the fingers
il braccio/le braccia - the arm/the arms
il ginocchio/le ginocchia - the knee/the knees
l’uovo/le uova - the egg/the eggs
il lenzuolo/le lenzuola - the bed sheet/the bed sheets

The plural of l’uomo (the man) is gli uomini (the men).