It's "a'.
The rule is NOT that you use "an" before a vowel, "a" before a consonant. It is that you use "an" before a vowel SOUND, "a" before a consonant SOUND.
the U in this cast makes the "y" sound aka a consonant sound so you use "a"
It's "a'.
The rule is NOT that you use "an" before a vowel, "a" before a consonant. It is that you use "an" before a vowel SOUND, "a" before a consonant SOUND.
the U in this cast makes the "y" sound aka a consonant sound so you use "a"
It's "a'.
The rule is NOT that you use "an" before a vowel, "a" before a consonant. It is that you use "an" before a vowel SOUND, "a" before a consonant SOUND.
the U in this cast makes the "y" sound aka a consonant sound so you use "a"

You must have went to a pretty bad school if they said "an unicorn" was the correct way.....
Or perhaps they are not native English speakers.
Or perhaps they are not native English speakers.