Note the highlighted syllables in the following words:
(1) dad
(2) pais
When performing the syllabic division of these words, we have:
(1) dad
(2) pa-is
Realize that while in "father" the i remains the same syllable that the The; in “country”, it separates, forming an independent syllable. But do you know why? It is due to the fact that, in (1), the i is semivowel and, in (2), it is a vowel. Thus, we can define that:
Vowels: they are the strongest vowel sounds that provide the basis for the syllable's existence. Analyzing the words below, we notice that the highlighted vowel has a greater sound in the syllable than the other phonemes that compose it:
gloryThe
mãand
viu
forThei
Semivowels - The vowels i and u they are called semivowels when they accompany another vowel and, therefore, they are asyllabic. See how these glides sound less when accompanied by a vowel:
Pani
whatuart
IMPORTANT! This classification refers to the phonemes /y/ and /w/ usually represented by the letters i and u, but they may appear with other orthographic representations in certain vowel clusters:
/?y/ - vin
/ãw/ - hand
/w?y/ - someman
/wãw/ - rinsewhoa
By Mariana Rigonatto
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/portugues/o-que-e-semivogal.htm