The rules of how to form the definite article "the"

The definite article agrees with its noun in number and gender:

il (the) is used with masculine, singular nouns: il ragazzo. - the boy

la (the) is used with feminine singular nouns: la ragazza. - the girl

l' (the) is used instead of il or la when the next word begins with a vowel:

l'amico, l'amica, l'americano, l'americana.

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Nouns ending in -o are nearly all masculine. il ragazzo.

Nouns ending in -a are generally feminine. la ragazza.

Nouns ending in -e are masculine or feminine. il padre. la madre. - the father, the mother.

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il becomes i (the) in the plural. i ragazzi.

la becomes le in the plural. le ragazze.

l' (before a masculine noun) becomes gli in the plural. gli amici. gli americani.

l' (before a feminine noun) becomes le in the plural. le amiche. le americane.

nouns ending in -o or -e form their plural by changing -o or -e to -i. i ragazzi, i padri. le madri.

nouns ending in -a form their plural by changing -a to -e. le ragazze.

lo (the) is used with masculine nouns which begin with s plus consonant, or with z, or with y:

lo specchio, lo studente, lo zero.

gli is the plural of lo. gli specchi, gli studenti, gli zeri.