Italian words are divided into syllables as follows:
A single consonant goes with the following vowel.

Italian English
ca–sa house
po–si–ti–vo positive

Double consonants are divided.

Italian English
bab–bo dad
ros–so red
bel–lo beautiful
at–to act

Two consonants, the first of which is l, m, n, or r, are divided.

Italian English
al–ber–go hotel
con–ten–to contented
am–pio ample
for–tu–na fortune

Otherwise, a combination of two consonants belongs to the following syllable.

Italian English
ba–sta enough
fi–glio son
pa–dre father
ba–gno bath
so–pra above
sa–cro sacred

The first of three consonants, except s, goes with the preceding syllable.

Italian English
sem–pre always

fel–tro felt
mem–bro member
men–tre while

BUT

Italian English
fi–ne–stra window
pe–sche peaches
mi–ne–stra soup
mo–stro monster

Diphthongs and triphthongs are never divided.

Italian English
nuo–vo new
mie–le honey
per–fi–dia spite
uo–mo man
mai never
lin–gua language
suoi his
pi–gliai I took

Diphthongs may occur in stressed or unstressed syllables. However, when a diphthong is broken by stress (the vowel i or u directly bears the stress), then the two vowels break into separate syllables.

Italian English
mi–o mine
tu–o yours
spi–a spy
ma–ni–a mania
rin–vi–o postponement
te–ra–pi–a therapy
al–le–gri–a joy
far–ma–ci–a pharmacy