How to Decide between “a” and “An”
Some non-native English speakers have trouble determining which indefinite article should be used when writing a sentence. Fortunately, the rules for this are quite simple.
The basic rule
The basic rule for determining which indefinite article to use is to look at the first letter of the word which comes after it. If that letter makes a consonant sound, a should be used. For example, you would write “a cat,” and not “an cat.”
Note that the vowel u sometimes makes a y sound. In these cases, a should be used. It is not the letter itself, but the sound it makes, that matters. “A unicorn” is correct; “an unicorn” is not.
If the word after the article starts with a vowel sound, you should use an. You would write “an umbrella,” and not “a umbrella.”
If a word begins with a silent h which is followed by a vowel, an should still be used. “An hour” is correct; “a hour” is not.
You need to be careful to check that the article you have used is still correct if you rewrite the sentence. For example, if you decide you want the reader to know that an umbrella is green, you would have to re-word “an umbrella” as “a green umbrella.” “An green umbrella” is incorrect because a word beginning with a consonant sound immediately follows the article in the revised phrase.
The special case of “h+vowel” words
There are some “h+vowel” words for which it is up to the writer whether a or an should be used. First, let’s look at such words for which the writer does not have this option:
- As stated before, words that begin with a silent h followed by a vowel have to be preceded by an.
- For words that begin with an h that does make a sound, a must precede it when the first syllable of the word is stressed (e.g., horse, happy, hospital).
A writer can use either a or an when neither of these points is true — that is, when the h is pronounced and the first syllable of the word is not stressed (e.g., historical, hypnosis).
To illustrate, both of the following sentences are correct (though the first is much more common):
“He had me under some kind of a hypnosis.”
“He had me under some kind of an hypnosis.”