pronunciation.diphthongs
Diphthongs: one syllable, two vowels
Weak (i, u) + strong (a, e, o) or weak + weak fuse into one syllable: bien, bueno, ciudad, aire. Strong + strong stay separate: le-er, ca-os.
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Explanation
Fourteen combos glide: ia ie io ua ue uo (rising: tierra, fuego), ai ei oi au eu ou (falling: aire, deuda), iu ui (ciudad, cuidado). They count as ONE syllable for stress rules.
An accent on the weak vowel breaks the diphthong into two syllables — that's hiatus (día, país) — see the hiatus rule.
Why this matters mechanically: a diphthong counts as a single syllable for the stress rules, so vie-ne and bue-no are two syllables, not three. The fusion only breaks when a written accent lands on the weak vowel (i or u) — that is hiatus, and it is exactly what the tilde in día or país is signalling.
Examples
Well, there's time on Wednesday.
Region: global
Europe and Asia.
Region: global
'Bueno' and 'tierra' have a diphthong; 'día' and 'país' don't.
Region: global