Definite articles refer to specific nouns (the).

• Il: used before masculine singular nouns starting with a consonant. Plural: I.
• Lo: used before masculine singular nouns starting with s+consonant, z, y, ps, pn. Plural: Gli.
• L': used before masculine singular nouns starting with a vowel. Plural: Gli.

Feminine nouns use different articles:

• La: used before feminine singular nouns starting with a consonant. Plural: Le.
• L': used before feminine singular nouns starting with a vowel. Plural: Le.

Indefinite articles refer to non-specific nouns (a/an). They only have singular forms.

• Un: masculine consonant or vowel.
• Uno: masculine starting with s+consonant, z, etc.
• Una: feminine starting with a consonant.
• Un': feminine starting with a vowel.