The choice between a and an is based on sound, not spelling. Use an before a word that begins with a vowel sound, and use a before a word that begins with a consonant sound.
This is why some phrases look surprising on the page. The word hour starts with the letter h, but the h is silent in many accents, so the word begins with an “ow” sound. That makes an hour correct. On the other hand, university starts with the letter u, but it often begins with a “yoo” sound, which is a consonant sound. That makes a university correct.
- an honor (the h is silent, it starts with an “on” sound)
- a one time offer (it starts with a “w” sound)
- an MBA (it starts with the vowel sound “em”)
- a European trip (it starts with a “yoo” sound)
If you are unsure, say the phrase out loud and listen to the very first sound of the next word. Choosing by sound makes your writing match natural speech, even when spelling suggests the opposite.