Note the pronunciation of the word below:
VANITY
There are two possibilities for the pronunciation of this word:
go - from - from
or
va - u - da - from
This is due to the existence of a vowel encounter between a vowel and a semivowel that together make up the same syllable, that is, they make up a DIPHTHONG in:
vthere-da-de
But sometimes this encounter can be pronounced as a GAP, transforming the semivowel i in a vowel that supports a different syllable:
vThe -i -of -of
Note that, in this change, the vowel “a” makes up one syllable, and the vowel “i” makes up another different syllable.
This phonological phenomenon is possible and receives a specific name in Portuguese, it is the DIERESIS. Thus, we can define that:
→ Dieresis it is the passage from a semivowel to a vowel, transforming a diphthong into a hiatus.
Examples:
betrayal (diphthong)
trThe-i-tion (hiatus)
falls (diphthong)
çThe-i (gap)
sau-do-so (diphthong)
sThe-u-do-so (hiatus)
By Mariana Rigonatto
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/portugues/o-que-e-dierese.htm