A, An

Use a when the first letter of the word following has the sound of a consonant. Keep in mind that some vowels can sound like consonants, such as when they're sounded out as individual letters. Also, some letters, notably h and u, sometimes act as consonants (home, usual), other times as vowels (honest, unusual).

Examples:
a yearning
a hotel
a U-turn (pronounced "yoo")
a NASA study

Use an when the first letter of the word following has the sound of a vowel.

Examples:
an unfair charge
an honor (the h is silent)
an HMO plan (H is pronounced "aitch")
an NAACP convention (the N is pronounced "en")


Confusing Words and Homonyms

Letter A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z