You probably think that “a” vs. “an” is a grammar rule you mastered long ago. But as it turns out, the concept is more complicated than you may think. The odds are that you’ve been using “an” in a sentence sometimes when you should have used “a” and vice versa.

“A” and “an” are indefinite articles, which are used to indicate how many of a certain noun you’re talking about. In other languages, such as Spanish, indefinite articles also indicate the gender of a noun. Luckily, in English, articles are much simpler. You only have four choices: “a,” “an,” “the” and “some.” “A” and “an” are used with singular nouns, and they are indefinite, meaning they don’t refer to a particular noun. That is the job of “the,” English’s only definite article.

When to Use “An” in a Sentence

You’re supposed to use “a” for words that start with consonants and “an” for words that start with vowels, right? Well, not always. Here’s a surprise: both “a unicorn” and “an umbrella” are correct. The real rule is that you have to use “an” in a sentence when a word has a vowel sound at the beginning. For words with vowels at the beginning that sound like consonants, such as the “u” in unicorn, use “a” instead. (For a list of vowel examples, see below.)

Believe it or not, there are other rules for using “a” vs. “an” correctly.

Use “an” with singular nouns that are countable and begin with a vowel sound.

Examples:

That is an alligator.

He rented an armadillo costume.

She sent her an invitation.

When an adjective comes between the indefinite article and the noun, make sure the article matches the sound of the adjective it proceeds. Use “an” with adjectives that begin with vowel sounds.

Examples:

I bought an expensive raincoat.

Mosquitos make an irritating noise.

She earned an excellent score on the test.

Some words begin with consonants that are silent, like “hour,” and other words sound like they begin with a vowel when they really start with a consonant.

Examples:

He was just asking an honest question.

She had to wait for an hour to see them.

They made an honest mistake.

Other times, you may be referring to just a singular letter or number. Make sure it matches with its indefinite article. Letters and numbers that sound like they start with a vowel should be proceeded by “an.”

Examples:

Even though he studied, his final grade on the exam was an F.

Her handwriting was so sloppy, they couldn’t tell if she spelled her name with an M or an N.

When the group was playing Yahtzee, he rolled an eight.

When to Use “A” in a Sentence

The other indefinite article is "a." As a general rule, chose "a" when the word that comes after it sounds like it starts with a consonant, even if it's actually a vowel. Sometimes weird things like that happen. English is complicated.

Use “a” in front of singular nouns that begin with consonants. These nouns should also be countable, meaning there could be more than one in other situations.

Examples:

She is a firefighter.

He uses a jump rope.

They saw a flower in the garden.

Indefinite articles have to match the sound of the word that comes after them, even if those words are adjectives that come between the article and its noun. Make sure you use “a” with adjectives that begin with consonant sounds.

Examples:

It was a stupendous surprise.

He took a picture of a tall elephant.

She examined a scaly lizard under her magnifying glass.

Use “a” in front of words that start with vowels that sound like consonants. Such vowel examples include the “u” in “university” or the “E” in “European.”

Examples:

She has a European Union passport.

They go to a university like ours.

The candy bar only costs a euro.

Use “a” before single letters and numbers that begin with consonant sounds, even if they are vowels or are spelled out with a vowel at the beginning.

Examples:

He gave me a $1 bill.

She made a U-turn.

Sometimes, Both "A" and "An" Work

In some situations, “a” vs. “an” is a question of where you’re from, or, more specifically, your accent.

Americans and speakers of British English are sometimes separated by a common language. One case in point is this discrepancy with “herb” and “hospital.”

In British English, the “h” in “herb” is pronounced, but in most American English accents, the same “h” is silent.

So, in the UK, it would be acceptable to say: I ordered a herb salad.

But in the United States, most people would say: I made an herb vinaigrette.

The same is true for the word “hospital” and practically any other word that begins with a silent “h” in American English.

British English speakers would say: She works at an hospital.

But American English speakers would say: He was born in a hospital.

Sometimes, Neither "A" Nor "An" Works

Only use an indefinite article for countable nouns. There are some nouns that can’t be counted, at least according to grammarians. These nouns include air, advice, information, fun and salt.

Wrong: She gave me an advice.

Right: She gave me some advice.

Wrong: He breathed an air.

Right: He breathed the air.

Wrong: They asked for an information.

Right: They asked for information.

Vowels Vs. Consonants

You probably already know the basic differences between vowels and consonants. Vowel examples in English include a, e, i, o, u and sometimes y. The rest of the letters in the alphabet are consonants. But do you know why they’re different?

It all has to do with the way your mouth and throat forms the sounds. When you pronounce vowels, your mouth and throat stay open, but consonants are formed by the different ways you stop or close that air passage with your teeth, tongue and lips.

The letter “y” is sometimes a vowel because your mouth treats it differently depending on the word. If you’re saying a word like “hymn,” the air passage stays open for the sound to come through. But if you’re saying a word like “yeoman,” your lips and tongue get involved, changing the airflow to make the “yuh” sound that starts off the word.

Other Grammar Rules to Watch Out For

Grammar can be much trickier than it looks because it seems like there are so many rules. But with a little knowledge and a lot of practice, you can become a grammar master and start being the one who corrects your friends.

Some of the most common grammar mistakes are just misspellings. Lean how to spell each word the right way, and your grammar will sparkle in no time.

Their, There, They’re: These three words mean three different things.

Use "their" if you mean something belongs to someone. Example: It's their house.

Use "there" when implying location. Example: It's that house over there.

Use "they're" to make a contraction of "they" and "are." Example: They're moving into the house across the street.

Your and You’re: Similar to the "their, there, they’re" problem, "your" and "you're" have two different functions.

"Your" is a possessive pronoun. Example: Is that your car?

"You're" is a contraction of "you" and "are." Example: You're the person driving us to school?

Its and It’s: These two words also have different functions.

"Its" is a possessive pronoun. Example: Let the dog chew on its toy.

_"_It's" is a contraction of "it" and "is." Example: Hurray! It's Christmas!

To and Too: Here is another instance of two words that sound alike but have separate meanings. Be careful: adding one letter or leaving it off can really confuse your reader.

"To" is a preposition. It can be directional or it can indicate time, among its numerous other uses. Example: We're going to the movies tonight.

"Too" is an adverb that can mean "also" or that something is in excess. Example: There is too much pepper on my potato salad. Example: Mine, too!

Me and I: This is one that might be tripping you up. You may have been told to use "she and I" instead of "she and me," but that advice could have led you astray. "She and I" is actually incorrect if you're using it in the predicate, or the end of the sentence that is being acted upon by the noun.

Wrong: She and me went to the party.

Right: She and I went to the party.

Wrong: He went to the party with Lucy and I.

Right: He went to the party with Lucy and me.

When in doubt, you can replace the offending word with another you know is right.

Right: He went to the party with us.

You can also take out the other person to see if the sentence sounds right with "me" or "I"

Wrong: He went to the party with I.

Right: He went to the party with me.

“Alot” and “a lot”: This is just a standard misspelling. "A lot" is two words, meaning there is a great quantity of something.

Wrong: I love you alot.

Right: I love you a lot.

Loose and Lose: Another common misspelling, "loose" and "lose" have unrelated meanings. "Loose" is an adjective, meaning something it baggy or not tight. "Lose" is a verb that means to misplace or be defeated.

Wrong: My pants are too lose.

Right: My pants are too loose.

Wrong: A zebra escaped from the zoo, and now it's on the lose.

Right: A zebra escaped from the zoo, and now it's on the loose.

Then and Than: "Then" and "than" may sound very similar, but they are different parts of speech with dissimilar functions.

"Than" is usually used in comparisons. Example: My turtle runs faster than your snail.

"Then" tends to indicate time or subsequent action. Example: I ate breakfast, and then I went to school.

Less and Fewer: Only brainiacs know this one. Both of these words are used for comparisons, but do you know when to use which?

Use "less" when you're talking about something collective or something that can be in a mass. Example: There is less sand in my shoes than in my shorts.

Use "fewer" when you're describing something that can be numbered. Example: I ate fewer cookies than my brother did.

Who and Whom: This one frequently trips up grammar novices. Both words replace nouns, but when should you use them?

"Whom" is always going to be the object of a very or preposition. If you want to use "whom," try replacing the word with "him" or "her." If it fits, "whom" is the correct word to use. If it doesn't, go with "who."

Wrong: Whom ate my sandwich?

Right: Who ate my sandwich?

Wrong: To who did you send that postcard?

Right: To whom did you send that postcard?

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About the Author

Rebecca Renner is a teacher and college professor from Florida. She loves teaching about literature, and she writes about books for Book Riot, Real Simple, Electric Literature and more.