a/an
A and an are indefinite articles; indefinite indicates that whatever the article points to belongs to the category named but is not a specific member of it.
The operator dialed a number. [Not a specific number but just some number. Contrast "The operator dialed the number"; the definite article, the, indicates that a specific number is referred to.]
The sound (not the letter) that follows a or an determines which should be used: Use an before vowel sounds (the sounds of a, e, i, o, or u); use a before any consonant sound.
The films plot is summarized in a one-page review. [Say the word one aloud; the first sound is of a consonant, w (as in won), even though the first letter is a vowel, o. ]
He arrived an hour early : [Although the h in hour is a consonant, the h is silent and the word begins with a vowel sound.]
It was a historic event. |The consonant h is pronounced, so a precedes the consonant sound.]
An unexpected scholarship offered Carlos a unique opportunity. [Unexpected begins with a vowel sound (u); unique begins with a consonant sound (y).]
He bought an SlR camera. [The letter S is here pronounced as ess, which begins with a vowel sound.]
A sonar operator detected the wreck. [Sonar begins with a consonant sound.]
