Latin has the following six diphthongs
combinations of two vowel sounds
that were collapsed together into a single syllable
ae as in ai in aisle: cārae, saepe
au as ou in house: aut, laudō
ei as ei in reign: deinde
eu as Latin e + u
pronounced rapidly as a single syllable: seu.
oe as oi in oil: coepit, proelium
ui as in Latin u + i
spoken as a single syllable like Spanish muy
or like English gooey
pronounced quickly as a single syllable
This diphthong occurs only in: huius, cuius, huic, cui, hui.
Elsewhere the two letters are spoken separately as in: fuit, frūctuī