Honestly, if you ask a room full of English teachers what part of speech is a or an, you might actually start a small, nerdy riot. Most of us were taught back in elementary school that "a," "an," and "the" are articles. That’s true. But "article" isn't technically one of the original eight parts of speech you memorized alongside nouns and verbs. So, where do they actually live?
Basically, "a" and "an" are specialized adjectives. Or, if you want to sound like a modern linguist, they are determiners.
Think about it this way. When you say "a cat," you aren't talking about any specific feline. You're talking about one, non-specific member of that group. You're modifying the noun "cat." Because they modify nouns, they function like adjectives. But they are weird adjectives. You can’t say "a more cat" or "the very a cat." They have their own rules.
The Grammar Identity Crisis
Most traditional grammars categorize "a" and "an" as indefinite articles. They belong to the broader family of adjectives because their sole purpose in life is to provide more information about a noun. Specifically, they tell us about the noun's "definiteness."
In the 19th century, grammarians like Lindley Murray—who basically wrote the book on English grammar for that era—grouped them squarely with adjectives. But as language study evolved, linguists realized that articles do something different than words like "blue" or "stinky." You can pile up adjectives: "the big, smelly, green trash can." But you can't pile up articles. You’d never say "the a trash can."
This is why modern linguistics created a separate bucket called determiners.
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Why "Determiner" is the term you’ll hear now
If you pick up a university-level textbook today, like The Cambridge Grammar of the English Language by Rodney Huddleston and Geoffrey Pullum, they’ll call "a" and "an" determiners. Determiners are a functional category. They "determine" the reference of a noun.
- Articles: a, an, the
- Demonstratives: this, that, those
- Possessives: my, your, his
- Quantifiers: some, every, many
All these words compete for the same slot in a sentence. You can have "my car" or "a car," but not "my a car." Because they occupy that specific grammatical "slot" right before the noun (or before the adjectives modifying the noun), they are distinct from standard descriptive adjectives.
When to Use A vs. An: The Sound Rule
This is where people get tripped up. It isn't about the letter. It’s about the sound.
We use "a" before consonant sounds and "an" before vowel sounds. Sounds simple, right? It usually is, until you hit the letter "H" or "U."
Take the word "university." It starts with a "U," which is a vowel. But when you say it, the first sound is a "Y" sound (yoo-ni-ver-si-ty). "Y" is a consonant sound. So, you say "a university."
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Now look at "hour." It starts with "H," a consonant. But the "H" is silent. You’re actually saying "our." That’s a vowel sound. So, you say "an hour."
The "An Historic" Debate
This is a classic "expert" trap. You’ll see some writers use "an historic event." Is it wrong? Sorta. In modern American English, we usually pronounce the "H" in historic. Since the "H" is audible, "a historic" is the standard. However, in some British dialects or older forms of English where the "H" was dropped (like 'istoric), "an" made sense. Today, using "an" before "historic" often sounds a bit pretentious or dated, though it’s technically accepted in some formal styles.
The Sneaky Power of the Indefinite Article
Most people think "a" or "an" just means "one." While that’s where the words come from—"a" actually evolved from the Old English word "an," which meant "one"—they do a lot more work than just counting.
They signal new information.
Look at this: "A man walked into a bar. The man ordered a drink."
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The first time we mention the man, we use "a." This tells the listener, "Hey, I'm introducing a new character you don't know yet." Once he's established, we switch to "the." If you said "The man walked into a bar" as your opening sentence, your friend would ask, "Wait, what man?"
It’s a subtle piece of mental software. "A" and "an" prepare the brain to create a new "file" for a person or object. "The" tells the brain to look for an existing file.
Rare Cases: When "A" Becomes Something Else
Language is messy. Sometimes "a" isn't a determiner at all.
Ever go "a-hunting" or "a-climbimg"? In these old-fashioned or dialect-heavy phrases, "a" is actually a preposition in disguise. It’s a shortened version of the Old English "on."
Then there’s the use of "a" to mean "per."
"I work forty hours a week."
In this context, grammarians argue whether it’s still an article or if it’s functioning as a preposition meaning "in each." Most stick with article, but the function is definitely unique.
Common Mistakes to Avoid
- The Acronym Trap: Do you say "a MRI" or "an MRI"? Read it out loud. "M" is pronounced "em." That starts with an "E" sound. So, it’s "an MRI." Similarly, it’s "an FBI agent" but "a Federal Bureau of Investigation agent."
- The "Unique" Exception: "A unique opportunity." Even though it starts with "U," it’s that "Y" sound again. Never use "an unique." It sounds clunky because it is.
- Omission in Headlines: Journalists often cut "a" and "an" to save space. "Man Bites Dog" instead of "A Man Bites a Dog." This is fine for news, but in your writing, omitting them can make you sound like a robot or someone who hasn't quite mastered English syntax.
Actionable Takeaways for Your Writing
If you want to master the use of these tiny but mighty words, keep these tips in your back pocket:
- Trust your ears, not your eyes. If you’re unsure whether to use "a" or "an," say the sentence out loud. If it feels like there’s a "hiccup" in your throat (like saying "a apple"), you probably need the other one.
- Use "a/an" for generalizations. If you say "A doctor should be kind," you mean any doctor. If you say "The doctor should be kind," you’re talking about a specific person, perhaps the one currently holding a needle over your arm.
- Check your acronyms. This is the #1 place where professional writers make mistakes. An HMO, a UFO, an RSVP.
- Watch for "H" words. A hotel, a hero, a habit. But... an honor, an heir, an hour. The silence of the "H" is the deciding factor.
Understanding what part of speech is a or an helps you see the skeleton of the English language. They aren't just filler words; they are the pointers that help your reader navigate the world you're building on the page. Use them to introduce new ideas and keep your prose flowing naturally.