why w and y are called semi vowels
Semivowel In phonetics and phonology, a semivowel or glide, also known as a non-syllabic vocoid, is a sound that is phonetically similar to a vowel sound but functions as the syllable boundary, rather than as the nucleus of a syllable.[1] Examples of semivowels in English are the consonants y and w, in yes and west. Written /j w/ in IPA, y and w are near to the vowels ee and oo in seen and moon, written /iː uː/ in IPA.
Spanish orthography Spanish uses only the acute accent, over any vowel: á é í ó ú. This accent is used to mark the tonic (stressed) syllable, though it may also be used occasionally in monosyllabic words to distinguish homophones such as si ('if') and sí ('yes'). The only other diacritics used are the tilde on the letter ⟨ñ⟩, which is considered a separate letter from ⟨n⟩, and the diaeresis used in some instances over ⟨u⟩ in words like bilingüe ('bilingual').