When To Use “A” vs. When To Use “An” There are all sorts of grammar rules out there. But, one of the most basic is determining when you should use the very first letter of the alphabet. Do you know when to use a vs. when you should use an? Let’s break this one down! A vs. an The rule is: Use an before a word beginning with a vowel sound (not letter). It doesn’t matter how the word is spelled. It just matters how it is pronounced. Use a before a word with a consonant sound as well as y and w sounds. Which means it is not unusual to find a before a word starting with a vowel or an before a word starting with a consonant. Get that essay, email, or letter to Nana over the finish line with a little writing help from Grammar Coach™. Get grammar check, spelling help and more free! So, we say an MBA, an hour … but a BA, a horologist. You say either an historical event or a historical event, according to whether you pronounce the h. If the h- is sounded (you make the “ha” sound), then a is the proper form. Now that you’ve got that figured out, are you ready to tackle affect vs. effect?