Here's why you might use "an" before a word that doesn't start with a vowel letter:
* Silent "h": Words like "hour", "honour" (British English), and "heir" start with an "h", but the "h" is silent. They have a vowel sound at the beginning, so you'd use "an":
* "It took an hour to get there."
* "He's an heir to the throne."
* Words that start with a vowel sound: Even if a word begins with a consonant, if it sounds like it starts with a vowel, you would use "an".
* "An umbrella" (the "u" has a "yoo" sound)
* "An FBI agent" (the "F" has a "uh" sound)
* "An honest person" (the "h" is silent)
Let me know if you have more questions!