a, an

The articles a and an are not interchangeable.

A is used before words beginning with consonant sounds.

• a historic event (historic has a hard h sound)
• a one-time payment (one has a hard w sound)
• a United Airlines pilot (United has a hard y sound)
• a horror movie (horror has a hard h sound)
• a xylophone player (xylophone has a z sound)

An is used before words with vowel sounds or soft/silent h sounds.

• an x-ray technician (x-ray is said with a soft e like ex)
• an emergency measure (emergency has an e sound)
• an honorary degree (honorary has a soft/silent h sound)
• an NAACP attorney (NAACP begins with n, pronounced en and begins with an e sound)
• an 18-year-old (18 begins with a long a sound ate)
• an apple pie (apple begins with a short a sound ant)
• an Ypsilanti business (Ypsilanti begins with a short i sound like igloo)