What is the exception to rule in English grammar with word an?

The exception to the rule with the word "an" is when it comes before a word that begins with a consonant sound, even if the word itself is spelled with a vowel.

Here's the breakdown:

* The Rule: "An" is used before words that start with a vowel sound. For example: "an apple," "an orange," "an hour."

* The Exception: "An" is used before words that start with a consonant sound but are pronounced with a vowel sound. For example:

* "an honest man" (the "h" is silent, so the word begins with a vowel sound)

* "an umbrella" (the "u" is pronounced like the vowel sound in "uh")

* "an hour" (the "h" is silent, so the word begins with a vowel sound)

This is a tricky area in English grammar because it relies on how the word sounds, not just how it's spelled.