An is a small but important word called an article that we use instead of “a” before nouns that start with vowel sounds. You say an apple, an elephant, an idea, an orange, and an umbrella because each of these words begins with a vowel sound.

The choice between an and a depends on sound, on how the word actually sounds when you say it. You write an hour because the h is silent and the word sounds like it starts with ow. But you write a house because you actually pronounce the h sound at the beginning.

Using an instead of a makes your speech flow more smoothly. Try saying “a apple” out loud and you'll hear how awkward it sounds compared to “an apple.” Your mouth has to work harder to separate those two vowel sounds.

The rule seems simple, but English can be tricky: you say a uniform (not an uniform) because uniform starts with a y sound, and you say an MBA because MBA starts with an em sound. Listen to how words actually sound when you say them, and you'll usually get it right.