Does any word misuse cause more consternation than the simple flipping of articles a and an? The rule is simple: a comes before words beginning with a consonant or consonant sound; an comes before words beginning with a vowel or vowel sound.
Incorrect: Dakota ate a orange.
Correct: Dakota ate an orange.
Incorrect: Gregory became an zombie.
Correct: Gregory became a zombie.
The vowel u often makes a consonant sound (yew) at the beginning of words such as university and unified. Therefore, a must proceed these words even though they start with a vowel.
Be aware of words which begin with silent letters, such as the h in herb or heir. Since the initial sound is a vowel, you must use an.
The sound wins every time. Yet another reason you should read your work aloud in the revision phase.