The use of "an" before words that begin with "w"

Hello guys, I have a question: one of my students asked me why we don't use the indefinite article "an" before the word waiter, that seems to begin with the letter "u". I really don't know how to explain this. I have searched on the site, but I didn't found an explanation.
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Avatar do usuário Artful Dodger 4500 6 13 103
Answering your doubt, W isn't either a vowel (a,e,i,o,u) or a silent h, the only two cases where you can use the article 'an'. Therefore, we say: a waiter, a year, a heart; and an equipment, an hour, an urgent question.

The exception to the rules above is the vowel u with the 'iu' sound, which behaves as a consonant, as in: university (a university), uniform (a uniform) and unicorn (a unicorn).

Avatar do usuário Dourado 1095 2 23
Not only "u", but any word that begins with a consonant sound, such as in "a European", "a one-foot tablet", or "a eulogy".

At the same time, sometimes you have to use an before a consonant (that has a vowel sound), such as in "an honor", "an MC", "an FBI agent"