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_.