I’m sure you’re pretty confident about this one, aren’t you?
It’s generally accepted that you use “an” before a word starting with a vowel, and “a” before a word starting with a consonant. Right?
But hold on a minute. What about “an hour” or “a unique opportunity”? “Hour” starts with a consonant, and “unique” with a vowel, so what’s going on here?
In fact, the correct rule is very simple. It’s not about the letter that starts a word, it’s about the sound.
If a word starts with a vowel sound, you use “an”. If a word starts with a consonant sound, you should use “a”.
“I spent an hour in a hotel.”
“A unicorn is an unusual creature.”
For this reason, “an hotel” is most definitely wrong, as is “an historic event” – unless, of course, you have a very severe dropped-aitches situation going on!