If a diphthong has to be read as two vowels, each of them producing a syllable, the diphthong-forming vowel (i or u) is marked with an accent mark. Compare:
secretaria – [se-kre-´ta-rja]
secretaría – [se-kre-ta-´ri-a]
Remember that the accent can be placed on the strong vowel of a diphthong (a, e or o), in this case the diphthong remains:
entráis – [ent-´rais]