Before explaining what changes in the acute accent, let us confirm two meanings: diphthong and hiatus. The first is the meeting of a vowel + a semivowel or vice versa, these being pronounced in the same syllable. The second is the sequence of vowels belonging to different syllables.
the acute accent stop existing in a few cases, let's see:
• Paroxytones:
1. in words paroxytones, that is, in words whose stress falls on the penultimate syllable, the open diphthongs ei and oi that were accented are no longer. This fact is justified by the existence of oscillation between opening and closing in the articulation of these words. Thus, some terms that are written in a way today take on new orthographic formats, such as: assembly, idea, boa constrictor, proteic, heroic, etc. Others, on the other hand, continue as they are: jail, flood, support, whale, eighteen, etc.
However, the acute accent remains in the oxytones (words whose stress falls on the last syllable) and stressed monosyllables with open diphthongs -éi, -éu or oi, followed or not by -s: roles, hero, remoi, rings, islets, hat, etc.
2. In paroxytone words with gaps formed with i and u, the vowel preceding them is part of a diphthong, that is, when they are preceded by a diphthong. Thus: ugliness becomes ugliness, baiúca becomes baiuca.
However, the vowels i and u, oxytone or paroxytone, continue to be accentuated if the vowel that precedes them does not form a diphthong: exit, caffeine, egoism, bay, jealousy, relapse, sandwich, Piauí, etc.
3. In verbs where the stressed accent falls on the root, with the consonants g or q preceding the stressed vowel u. This is the case of: arguir and redarguir: arguo, arguis, arguir, arguem, and so on.
By Sabrina Vilarinho
Graduated in Letters
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grave accent - Does the employment of the crase change with the new agreement?
Source: Brazil School - https://brasilescola.uol.com.br/acordo-ortografico/acento-agudo.htm