“A” and “an” are indefinite articles. They are used before singular and plural nouns, noun phrases and non-count nouns that sometimes require placing an adjective before the noun.
The choice of whether to use “a” or “an” is made based on the sound quality of the first letter in a word, not the written representation of it. If the first letter in a word makes a vowel sound, “an” is used. Consider the following examples where “a” goes before words that begin with a consonant sound and start with a consonant:
If the first letter in a word makes a consonant sound, “a” is used. Consider the following examples where “an” goes before the words that begin with a vowel sound and start with a vowel:
“A” and “an” are also used to talk about something general or to modify a non-specific noun. Consider the following examples:
Example 1: Margaret admired a hat she saw in the window display. (“A hat” means any hat, not a particular hat.)
Example 2: Sebastian ordered an egg sandwich for lunch. (“An egg” refers to any egg, not a particular type of egg, such as scrambled or poached.)
Exception: use “a” when “u” sounds like “y”
“A” is used when the vowel “u” sounds like the consonant “y.” Consider the following examples:
Example 3: A unicorn is a horse-like mythical creature with a single horn.
In Example 3, “a” is used because “unicorn” is pronounced, “you-ni-corn.”
Example 4: The USS Kitty Hawk, a United States Naval ship, was used to transport aircraft and cargo during World War II.
In Example 4, “a” is used before United States because it is pronounced with an emphasis that sounds like “younited.” Therefore, “a United States” is correct.
Exception: use “a” when “o” sounds like “w”
“A” is used when the vowel “o” sounds likes the consonant “w.” Consider the following example:
Example 5: Since 1958, Don Rickles has performed a one-man show.
“A” is used before “one” because it is pronounced, “won.” It is important to remember that a consonant sound does not always begin with a consonant.
Exception: use” an” with the silent “h”
The article “an” is used before a silent “h.” When a word begins with the letter “h” and does not have an audible “h” sound, the sound that follows the article is a vowel sound. Therefore, “an” is used. Consider the following examples:
Example 6: Robert received an honorable mention for his work.
In Example 6, “an” is used because “honorable” is pronounced, “on-ur-uh-bull.” Pronunciation, not spelling, changes the rule.
Example 7: It should take an hour to drive from the airport to the hotel.
In Example 7, “an” is used because “hour” is pronounced, “ou-er.” It is important to remember that a vowel sound does not automatically begin with a vowel.