There are two classes of words that have a variable accentuation: first, those in which an unaccented weak vowel is followed by an accented strong vowel, e.g. majestu^oso, majestu|oso; second, those in which an accented strong vowel is followed by an unaccented strong vowel, e.g. tra|e, tra^e.
Ex. Cre^es que la afe|an. Becquer.
Cre|es que suspirando pasa el viento, p. 171, XVI, l. 3.
Etymological conditions often determine whether or not a diphthong is formed.
ie and ue, derived from the Latin e and o respectively, form indissoluble diphthongs.
The ending -ión for substantives is usually a diphthong and rarely suffers dissolution.
Synaeresis may be employed to unite in a single syllable two contiguous vowels (unaccented weak + accented strong) that are separated on account of etymology, or, in the case of derivatives, analogy with the original word; but diaeresis is employed very rarely to dissolve a proper diphthongal combination (unaccented weak + accented strong).
For example, di|ario by analogy with día, and fi|ó from the Latin fidavit, have ordinarily the i in separate syllables, but a diphthong may be formed by synaeresis.
(3) When the two contiguous vowels are weak.
(a) Two contiguous weak vowels with the accent on the first form an indissoluble diphthong, e.g. mu^y.