This seems like an obvious one, right? “A” goes before words beginning with consonants (e.g., a mouse) and “an” goes before words beginning with a vowel (e.g., an octopus). However, there are a couple of exceptions dealing with how the beginning of the word sounds. For example, you use “an” before a silent h because the beginning of the word makes a vowel-type sound (e.g., an honest mistake). Another exception is when the letter “u” at the beginning of a word sounds like a y—a consonant sound. In this instance, “u” should be treated as a consonant and would have an “a” in front of it (for example, union and useful). Additionally, “o” sometimes sounds like a “w” and should have “a” in front of it (for example, “my son is a one-man wrecking crew).