The rule for using "a" and "an" is actually more complicated than you may have been taught.
A lot of people learned the rule that you put “a” before words that start with consonants and “an” before words that start with vowels, but it's actually more complicated than that. For example, here's Matthew with a question:
I've been wondering if it is actually “a hour” or “an hour.” “An hour” sounds more correct, but “a hour” reads more correct to me. I'm just curious what it should be.
The rule is that you use “a" before words that start with a consonant sound and “an” before words that start with a vowel sound.
Should You Use 'A' or 'An'?
So to answer Matt's question, “an hour” is correct, because “hour” starts with a vowel sound. People seem to ask most often about words that start with the letters H and U because sometimes these words start with vowel sounds and sometimes they start with consonant sounds. For example, it is “a historic monument” because “historic” starts with an H sound, but it is “an honorable fellow” because “honorable” starts with an O sound. Similarly, it is “a Utopian idea,” but “an unfair world.”
The letters O and M can be tricky too. Usually you put “an” before words that start with O, but sometimes you use A. For example, you’d use A if you were to say, “She has a one-track mind,” because “one-track” starts with a W sound. Similarly, you’d say, “She has an MBA, but chooses to work as a missionary,” because “MBA” starts with a vowel sound and “missionary” starts with a consonant sound.
Use “a” before words that start with a consonant sound and “an” before words that start with a vowel sound.
Other letters can also be pronounced either way. Just remember it is the sound that governs whether you use “a” or “an,” not the actual first letter of the word.
One complication is when words are pronounced differently in British English and American English. For example, the word for a certain kind of plant is pronounced “erb” in American English and “herb” in British English. So the proper form in America is “an herb,” and the proper form in Britain is “a herb.” In the rare cases where this is a problem, use the form that will be expected in your country or by the majority of your readers.
Next: 'A Historic' or 'an Historic'?