There are two indefinite articles in English: a and an. We use the indefinite articles a and an before nouns that refer to things that are new ideas, not already known, or that are not specific. We only use indefinite articles before singular, countable nouns.
Articles are a type of determiner, a part of speech which we use before a noun.
When to use Indefinite Articles
One very common situation when we use indefinite articles is when we are introducing a new idea that hasn’t been mentioned before. It is very common to use following there is. Here are a couple of examples:
There is a blue house on the corner of my street.
There is an umbrella in the closet.
I have an older brother.
We also use indefinite articles in situations where we don’t need a specific thing. Look at this example:
I need a glass of water.
The person speaking in the example above doesn’t need a specific glass of water. They are just thirsty, and any glass of water will be good. In situations like this, where we are not referring to a specific noun, we also use a or an.
The difference between A & An
The difference between a and an is based on the first sound of the next word. When the next sound is a consonant sound, we use a. When the next sound is a vowel sound, we use an. Here are some examples:
an apple
an egg
an ice cube
an onion
an umbrella
a bag
a dog
a girl
a house
a man
Note that some words are tricky because they may start with a letter that is a vowel, but with a sound that is a consonant. For example, the first letter in university and union is the vowel U, but the first sound is the consonant Y. Here are a few examples:
a European city
a euphemism
a union
a university
On the other hand, some words start an H, but the H is silent, so the first sound is a vowel. What matters is the sound, so we use an. Here are a few examples:
an hour
an herb
an honest mistake
An Historian or A Historian?
A weird example that frequently arises is the word historian. You may sometimes hear people say an historian. But is this correct? The short answer is probably not. The longer answer is it depends: If you speak with an accent that doesn’t pronounce the H in historian, then an historian is fine. But most accents today pronounce the H, and if you pronounce the H, you should probably say a historian!
The same rule applies for a historic event vs. an historic event and a hotel vs. an hotel. If you say the H, use a. If the H is silent in your accent, use an.