Singular a/an = un, uno, un', una
The Italian indefinite article (l'articolo indeterminativo) corresponds to English a/an and is used with singular nouns. It also corresponds to the number one.
Use uno for singular masculine nouns beginning with z or s+ a consonant. Exp: uno stadio (stadium)
Use un for all other singular masculine nouns. Exp: un orso, un naso (a bear, a nose)
Use una for singular feminine nouns starting with a consonant. Exp: una matita, una canzone (a pencil, a song)
Use un' for singular feminine nouns starting with a vowel. Exp: un'ape (a bee)