There are with regard to accent three possible conditions under which
two contiguous vowels may occur within a word.
a. The contiguous vowels may precede the accented syllable.
b. One of the contiguous vowels may be accented.
c. The contiguous vowels may come after the accented syllable.
a. Two contiguous vowels before the accent.
(1) Of the twenty-five possible combinations all are admissible in
diphthongs in a syllable preceding the accented syllable.
Ex. _Habrá po^esta_, p. 165, IV, l. 4.
(a) Diaeresis may be employed to dissolve the diphthong.
Ex. _Sobre una vi|oleta_, p. 169, XIII, l. 8.
b. One of two contiguous vowels accented.
(1) _When two contiguous vowel's are strong._
(a) There is no diphthong if one of two contiguous strong vowels
receives the accent.
Ex. _Chispë|ando el sol hiere_, p. 173, XXVI I, l. 17.
Ex. _Tú, sombra a|érea que, cuantas veces_, p. 170, XV, l. 7.
By synaeresis, however, a diphthong may be formed, especially in the
combinations á^o, á^e, ó^e--_c^a^o^s, c^a^e, ro^e_. But in order to
diphthongize oa, ea, and eo, when the accent naturally falls on the
first vowel, the accent must shift to the second, which is a dominant
vowel. Such diphthongization is harsh. For example, _loa_ would shift
the accent from o to a in order to form a diphthong. The accent would
also shift in _cre^a, fe^o_.
(2) _When one of the contiguous vowels is weak and the other strong._
(a) There is no diphthong if an accented weak vowel precedes a strong.
Ex. _Yo, que á tus ojos en mi agoní|a_, p. 171, XV, l. 18.
Synaeresis is, however, sometimes employed to overcome this rule.
The accent must then shift.
Ex. _Habi^a llegado una nave._ Calderón.
(b) There is no diphthong if an accented weak vowel follows a strong.
Ex. _¿Cómo puede re|ir?_ p. 182, XLIX, l. 4.
Synaeresis serves sometimes to overcome this rule. The result is
usually harsh.