This might look like a simple task, but many still get it wrong. The reason is that they focus on the first letter of the following word, instead of focusing on the sound of it. The correct way is to think about the sound that follows the indefinite article. If, after pronouncing it, the following word starts with a vowel sound, use "an." If it starts with a consonant sound, use "a." Examples: An hour (the "h" is silent, so it starts with a vowel sound) A university (the "u" is pronounced as "yoo," so it starts with a consonant sound) Keep in mind that this rule is based on sounds, not just the letters themselves.