- un is used with most masculine nouns (un ragazzo = a boy)
- uno goes with masculine nouns starting with z or s+consonant (uno zio = an uncle, uno sbaglio = a mistake)
- una goes with feminine nouns starting with a consonant (una ragazza = a girl)
- un’ goes with feminine nouns starting with a vowel (un’automobile = a car)
singular DEFINITE ARTICLES
- il is used with most masculine nouns (il ragazzo = the boy)
- lo is used with masculine nouns beginning with z or s+consonant (lo zio = the uncle, lo sbaglio = the mistake)
- la is used with feminine nouns (la ragazza = the girl)
- l’ is used instead of lo or la in front of vowels (l’animale = the animal)
PLURAL DEFINITE ARTICLES
- i goes with most masculine nouns starting with consonants (i ragazzi = the boys)
- gli is used before any masculine nouns beginning with a vowel, z or s+consonant (gli amici = the friends, gli zii = the uncles, gli studenti = the students).
- le is used with feminine nouns, even if they begin with a vowel (le amiche = the female friends, le madri = the mothers).
For more on nouns and how to use articles in Italian, go here and here.