Ask the wise old owl
Why can’t I say “an useful book”?
We learn that if the word directly after “a” begins with a vowel (a, e, i, o or u), “a” becomes “an”.
An old horse.
An enormous birthday cake.
However, in some cases where the word begins with “u”, this rule doesn’t apply because the sound is like the “y”, in “you” rather than the “u” in “umbrella”.
Therefore we have to say:
A used car
A useful book.
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