This document provides guidance on using "a" versus "an" in sentences. It states that "a" should be used when the following word begins with a consonant sound, while "an" should be used when the following word begins with a vowel sound. It notes there can be confusion when a word starts with a vowel but the sound is consonant, like "one," or starts with a consonant but the sound is a vowel, like "hour." It also mentions using "a" before words starting with "y" when "y" makes a consonant sound, like "yard." Examples are given demonstrating the rules in practice.