The Italian equivalent to the English a or an is the indefinite article un, uno, and una (un’).
The masculine forms are: un and uno.
The feminine forms are: una and un’.
Here are some grammar rules that govern indefinite articles:
Un is used for masculine nouns that start with a vowel and most consonants: un albero or un carciofo
ATTENZIONE! Sometimes students use the un’ form with masculine nouns that begin with vowels! The elision with nouns that start with a vowel is only for feminine nouns.
Uno is used before nouns that begin with s+consonant, ps-, gn-, y, and z: uno zio, uno psicologo, uno gnocco, uno yogurt.
Una elides with nouns that start with vowels and becomes un’: un’anatra, un’idiota (f.)
Una is used before all other feminine nouns: una macchina, una penna