Italian and English differ in their usage of adjectives. Italian descriptive adjectives are usually placed after the noun they modify, and with which they agree in gender and number.
COMMON ITALIAN ADJECTIVES ENDING IN -O
allegro
cheerful, happy
buono
good, kind
cattivo
bad, wicked
freddo
cold
grasso
fat
leggero
light
nuovo
new
pieno
full
stretto
narrow
timido
timid, shy
Adjectives ending in -o have four forms: masculine singular, masculine plural, feminine singular, and feminine plural. Observe how the adjectives nero and cattivo change to agree with nouns they modify.
ENDINGS OF -O ADJECTIVES
SINGULAR
PLURAL
il gatto nero (the black cat, masculine)
i gatti neri (the black cats, masculine)
la gatta nera (the black cat, feminine)
le gatte nere (the black cats, feminine)
il ragazzo cattivo (the bad boy)
i ragazzi cattivi (the bad boys)
la ragazza cattiva (the bad girl)
le ragazze cattive (the bad girls)
Note that when an adjective modifies two nouns of different gender, it retains its masculine ending. For example: i padri e le madre italiani (Italian fathers and mothers).
Although the majority of Italian adjectives have four forms (as in italiano, italiana, italiani, italiane) there are exceptions. Not all Italian adjectives have a singular form ending in -o. There are a number of adjectives that end in -e. The singular ending -e changes to -i in the plural, whether the noun is masculine or feminine.
ENDINGS OF -E ADJECTIVES
SINGULAR
PLURAL
il ragazzo triste (the sad boy)
i ragazzi tristi(the sad boys)
la ragazza triste (the sad girl)
le ragazze tristi (the sad girls)
ITALIAN ADJECTIVES ENDING IN -E
abile
able
difficile
difficult
felice
happy
forte
strong
grande
big, large, great
importante
important
intelligente
intelligent
interessante
interesting
triste
sad
veloce
fast, speedy
There are quite a few other exceptions for forming plural adjectives. For instance, adjectives that end in -io (with the stress falling on that i) form the plural with the ending -ii: addio/addii; leggio/leggii; zio/zii. The table below contains a chart of other irregular adjective endings you should know.
FORMING PLURAL ADJECTIVES
SINGULAR ENDING
PLURAL ENDING
-ca
-che
-cia
-ce
-cio
-ci
-co
-chi
-ga
-ghe
-gia
-ge
-gio
-gi
-glia
-glie
-glio
-gli
-go
-ghi
-scia
-sce
-scio
-sci
From:https://www.dbfei.com/Article/yidaliyu/201311/7724.html