In Italian, indefinite articles are words that take the meaning A or AN, when paired with a noun. As in English, indefinite articles refer to a general object or when the identity is unknown.
Indefinite articles have no plural form, but like other articles and adjectives, they must agree in gender with nouns. Like definite articles, indefinite articles always come before the noun.

Examples:​

• Marco mangia un'insalata. 

            Marco is eating a salad.

• C'è una donna fuori.

           There is a woman outside.

• C'è un albero grande nel giardino. 

           There is a big tree in the garden.

• Ti posso offrire un caffè?

            Can I offer (buy) you a coffee?

• Compro uno zaino. 

            I am buying a backpack.

In the above examples,  the articles un', una, un, uno refer to indefinite (non-specific) objects or unknown persons.

Rules for using indefinite articles:

Indefinite articles are not used with proper nouns (ex: Londra, Roma, Mario, Anna...) with the exception of brand names to refer to a specific product: (ex: una Ferrari, un Xbox...).

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Pratica!
​Review Gender of Nouns if necessary.