It’s just a single letter, but the differences regarding when to use a or an can be confusing.
Most of us learned to use an before words that start with vowels and use a everywhere else.
Honestly, this is a pretty good rule, and will serve you well most of the time.
The problem arises when you don’t realize that it matters how the word before the indefinite article (a, an) sounds, not just what it starts with.
You use a before words that start with a consonant sound and an before words that start with a vowel sound.
For example, The show starts in about an hour.
This example is correct because hour sounds like it starts with an o when it’s spoken.
So, remember to pay attention to not only the letter the word begins with, but also how it sounds when deciding whether to use a or an.
Now go forth and use words correctly!