I've asked elsewhere but have yet to receive an answer which is both logical and convincing so let me try here.

Just when is the letter Y counted as a vowel?

I was taught that a Y was a vowel when it added a syllable to a word (as in "brightly") or when it was the only vowel sound in the word (as in "fly"). This means that the Y in "boy" would NOT be a vowel. Unfortunately, I've also been instructed (more recently - maybe this is significant?) that the Y in "boy" IS a vowel since it alters the pronunciation of the O. The Y is pronounced as would an I in that place ("boy" = "boi"), hence its status as a vowel.

Any clarification would be appreciated.