Remember that in Italian, every noun is either masculine or feminine so the definite article, "the"
in English, is either masculine singular, masculine plural, feminine singular or feminine plural.
a. Masculine
Il : for masculine singular nouns which start with a consonant.
Il libro / the book
Il vino / the wine
Il tavolo / the table
L' : for masculine singular nouns which start with a vowel.
L' orologio / the watch
L'aereo / the plane
L'albero / the tree
Lo : for masculine singular nouns which start with:
s+
consonant
z
y
ps
gn
lo studente, lo scontrino / the student, the receipt
lo zaino, lo zio / the backpack, the uncle
lo yogurt
lo psicologo / the psychologist
lo gnomo / the gnome
I : for masculine plural nouns which start with a consonant.
I libri / the books
I vini / the wines
I tavoli / the tables
Gli : for masculine plural nouns which start with a vowel, s + consonant, z, y, ps, x or gn.
Gli orologi / the watches
Gli aerei / the planes
Gli alberi / the trees
Gli studenti / the students
Gli zaini / the backpacks
Gli yogurt / the yoghurts
Gli psicologi / the psychologists
Gli gnomi / the gnomes
b. Feminine
La : for feminine singular nouns which start with a consonant.
La penna / the pen
La camicia / the shirt
La sedia / the chair
L' : for feminine singular nouns which start with a vowel.
L' ape / the bee
L'insalata / the salad
L'ora / the hour
Le : for feminine singular nouns which start with a vowel or a consonant.
Le penne / the pens
Le camicie / the shirts
Le sedie / the chairs
Le api / the bees
Le insalate / the salads
Le ore / the hours
**From oneworlditaliano.com