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