(3) If the first of the contiguous vowels is strong, and the second weak and accented, they form separate syllables, as in raíz, baúl, roído. Contraction (synæresis) is rare and harsh in such cases: cf. Meléndez Valdés in the hendecasyllable,
Caído del cielo al lodo que le afea.
(4) If the first of the contiguous vowels is weak and the second strong, and the accent is on the weak vowel, they naturally constitute separate syllables, as in día, río, valúa, lloraríamos.
Synæresis is more frequent and less harsh here than in [(3)]; cf. Garcilaso:
Que había de ver con largo acabamiento.
Espronceda:
Los ríos su curso natural reprimen.
(5) If the first of two contiguous vowels is weak and the second is strong and accented, the vowels sometimes form one syllable and sometimes do not. Etymological conditions often determine the case; thus fió is a dissyllable, since it comes from a Latin source (fidavit) in which the i was in a syllable by itself, and bien is a monosyllable, since the i and e form a diphthong evolved out of a single Latin vowel (the ĕ of bĕne).
The chief cases are as follows:
(a) iè and uè from Latin e and o form diphthongs absolutely indissoluble: diente, muerte.