The difference between the articles “a” or “an” in a sentence is one of style and sound. When the next word begins with a consonant, use an “a” (a jackhammer, a tiger, a pack of wolves). When the following word starts with a vowel, use an “an” (an egg, an olive grove, an open wound).

Should I use “a” or “an” in a Sentence?

The exception to using “a” or “an” in a sentence is when the word after the article begins with a silent letter. Take for instance the word “hour”—though it starts with an “h,” it sounds like an “au.” Therefore, the proper phrase is “an hour.” Try saying the article plus the word out loud if you are unsure. Usually, your tongue will stumble over an error (in those last two words, “an” is correct because of “error,” though it has an “r” sound, still begins with a forceful “air” sound.)

Remember to base the rule about whether to use “a” or “an” in a sentence on the sound of the word, not necessarily the first letter. When the first letter is silent or begins with a “u” that sounds like “you” or an “o” that sounds like “w,” reverse the rule and use an “a.” So it should be “A unicorn is a one-horned pony.”

Fortunately, many spell-check programs will spot most of these errors, if you happen to make them. But the “a” vs. “an” rule is one of the simplest ones we’ve got, so it can’t hurt to learn it.

Originally posted 9/7/2011 and happily updated 10/25/2017. Thanks for reading!