In English the definite article (l'articolo determinativo) has only one form: the. In Italian, l’articolo determinativo has different forms according to the gender, number, and first letter of the noun or adjective it precedes.
i ragazzi e le ragazze (the boys and girls)
il quaderno e la penna (the notebook and pen)
Here are some rules for using definite articles:
1. Lo (pl. gli) is used before masculine nouns beginning with s + consonant or z.
2. Il (pl. i) is used before masculine nouns beginning with all other consonants.
3. L’ (pl. gli) is used before masculine nouns beginning with a vowel.
4. La (pl. le) is used before feminine nouns beginning with any consonant.
5. L’ (pl. le) is used before feminine nouns beginning with a vowel.
The article agrees in gender and number with the noun it modifies and is repeated before each noun.
gli italiani e i giapponesi (the Italians and the Japanese)
le zie e gli zii (the aunts and uncles)
The first letter of the word immediately following the article determines the article’s form. Compare the following:
il giorno (the day)
l’altro giorno (the other day)
lo zio (the uncle)
il vecchio zio (the old uncle)
i ragazzi (the boys)
gli stessi ragazzi (the same boys)
l’amica (the girlfriend)
la nuova amica (the new girlfriend)
In Italian, the definite article must always be used before the name of a language, except when the verbs parlare (to speak) or studiare (to study) directly precede the name of the language; in those cases, the use of the article is optional.
Studio l’italiano. (I study Italian.)
Parlo italiano. (I speak Italian.)
Parlo bene l’italiano. (I speak Italian well.)
The definite article is used before the days of the week to indicate a repeated, habitual activity.
Domenica studio. (I’m studying on Sunday.)
Marco non studia mai la domenica. (Marco never studies on Sundays.)
From:https://www.dbfei.com/Article/yidaliyu/201311/7738.html