a / an
The indefinite articles, chosen by sound, not by spelling.
Use a before a consonant sound and an before a vowel sound. The letter that starts the next word does not decide the article; the first sound does. Use a university (the word begins with a y sound, as in you), an hour (the h is silent), a historic event (the h is sounded), an FBI agent (the letter F is spoken “eff,” which begins with a vowel sound), a one-time offer (the word begins with a w sound).
Examples
UseShe bought a used car and an umbrella.
UseHe earned an MBA and later a PhD.
AvoidShe bought an used car.
AvoidHe waited a hour.
When the next word is an abbreviation or a numeral, say it aloud and match the article to the sound you hear: an 8-year-old, a 10-year-old.