What is the difference between using "a" or "an"?

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The indefinite articles a and an differ in usage based on the next word in the sentence.

General Rule:

Use a before a word that begins with a consonant and an before a word that begins with a vowel.

Examples:

a blanketa dolphina volcanoa countrya heron

...

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The indefinite articles a and an differ in usage based on the next word in the sentence.

General Rule:

Use a before a word that begins with a consonant and an before a word that begins with a vowel.

Examples:

a blanket
a dolphin
a volcano
a country
a heron

an idea
an Australian
an elephant
an iPad
an unrestrained child

Exception #1:

If the word begins with a consonant h that you really can't hear, and the first sound you actually hear is a vowel sound, use an as the indefinite article.

Examples:

an honor
an hour
an heir

Exception #2:

If a word begins with a u sound that sounds like the y in you, you should use a as an indefinite article. In this case, the y sound is actually a glide, which has properties of consonants, thus changing the speech pattern. This also applies to words that begin with an o that sounds like the w in won, where you should again use a as the indefinite article.

a unicorn
a utility
a ukulele
a one-sided argument

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Choosing between using the indefinite articles "a" or "an" depends on the word that they are placed in front of. If the indefinite article is placed before a word that begins with a consonant that is not silent, one uses the "a" form. You can see that in the sentence preceding this one, in which we write "a consonant." If the noun after the indefinite article begins with a vowel or a silent consonant, we use "an." That sound less peculiar to the English-speaking ear than having two vowel sounds back-to-back. In this case, we would, for example, write "an Eskimo" rather than "a Eskimo," as the latter would sound strange to us and be more difficult to say.

Both definite and indefinite articles can be difficult for non-native speakers whose languages don't use those forms, but the rules are by and large fairly straightforward.

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The indefinite articles [a type of adjective] a and an indicate a noun that is one of many. Some grammar books refer to them as "noun markers." That is, a common noun that is not specified as a certain one is preceded by a or an. When, for example, a person asks for "a pencil," it does not matter which pencil the person has, just as long as it is a pencil that works, probably (one of many).  However, if a person asks for "the pencil," he or she refers to a specific pencil, not just any one.  The article the, therefore, is termed a definite article.

In determining the use of an as opposed to a, whenever the noun that the article modifies begins with a vowel, or a vowel sound--e.g. an original idea, an hourly rate--Standard American English calls for the use of an. While there are some exceptions such as in the phrase, a unique experience, the noun that the indefinite article modifies begins with a cosonant or a cosonant sound, Standard American English calls for the use of a--e.g. a glorious sunset.

 

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In terms of how you use them in a sentence, there is no difference between these two words.  They are both indefinite articles that are used to refer to something that is not already known to the people who are talking.  So, in other words, if I say "I saw a cat" it means that you and I don't already know what cat I'm talking about.

As far as the difference between the two, the only difference is that "a" is used before words that start with a consonant sound and "an" is used before words that start with a vowel sound.  So we would say "a cat" but "an eagle."  We would also say "an hour" because that word sounds like it starts with an "o" rather than an "h."

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