In 2010, RAE made a number of changes in Spanish spelling. One of them was exclusion of the graphical accent in monosyllabic words with diphtongs or triphthongs: guion, truhan, fie, liais, etc.
First, RAE introduced strict rules to define what is a diphtong for spelling purposes:
an open vowel (/a/, /e/, /o/) followed or preceded by a closed unstressed vowel (/i/, /u/): estab-ai-s, conf-ia-r, d-ia-r-io, af-ei-tar, v-ie-nto, p-ie, d-oy etc (here hyphens do not mean anything at all, I was forced to use them to be able to emphasise word parts).
two different closed vowels (/i/, /u/): tr-iu-nfo, incl-ui-do etc.
Similarly, they defined what is a triphthong for spelling purposes:
- an open vowel between two closed unstressed vowels: conf-iái-s, act-uái-s, punt-uéi-s, g-uau.
As a result, a number of words that were previously two-syllable words became monosyllabic for spelling purposes (without any change in pronunciacion) and therefore do not need a graphical accent. These words are:
Las palabras afectadas por este cambio son formas verbales como
crie, crio, criais, crieis y las de voseo crias, cria (de criar);
fie, fio, fiais, fieis y las de voseo fias, fia (de fiar);
flui, fluis (de fluir);
frio, friais (de freír);
frui, fruis (de fruir);
guie, guio, guiais, guieis y las de voseo guias, guia (de guiar);
hui, huis (de huir);
lie, lio, liais, lieis y las de voseo lias, lia (de liar);
pie, pio, piais, pieis y las de voseo pias, pia (de piar);
rio, riais (de reír);
sustantivos como guion, ion, muon, pion, prion, ruan y truhan;
y ciertos nombres propios, como Ruan y Sion.
--Principales novedades de la última edición de la Ortografía de la lengua española (2010)