In Italian, the definite article generally corresponds to its use in English. In other words, when you need one in English, you probably need one in the Italian equivalent.

Il libro è sullo scaffale.     The book is on the shelf.

There are, however, some noteworthy differences.

Before Geographical Names

The definite article is used before geographical names such as countries, cities, continents, rivers, islands, regions, mountains, and lakes.

La francia è bella.     France is beautiful.

The definite article is not used after in if the geographical term is feminine, singular, and unmodified.

La Lombardia è in Italia. (feminine, singular, unmodified)	Lombardy is in Italy.
La Lombardia è nell'Italia settentrionale. (feminine, singular, modified)	Lombardy is in northern Italy.

The definite article is used before a proper name preceded by a professional or respectful title when speaking about the person.

La signora Romeo è di Roma.     Mrs. Romeo is from Rome.

When talking directly to the person, the article is omitted.

Signora Romeo, è di Roma Lei?     Mrs. Romeo, are you from Rome?

Colloquially, it is often used before names of women.

Hai visto la Laura ieri sera?     Did you see Laura last night?

The definite article is used before the names of languages.

Studio l'italiano e il francese, ma non studio l'inglese.

I study Italian and French, but I do not study English.

The definite article is used before abstract or concrete nouns that are used to express a general sense.

Mi piacciono i vini italiani.	I like Italian wines.
Lo zucchero è dolce.	Sugar is sweet.

The definite article is used when referring to parts of the body or articles of clothing.

Mi lavo le mani.     I wash my hands.

The definite article is used after the verb avere when giving a physical description.

Marcello ha gli occhi azzurri.     Marcello has blue eyes.