What is the reason of using an before vowels?

The reason for using "an" before vowel sounds, rather than "a", is to avoid a harsh sound.

It's not strictly about the letter, but the sound the word starts with.

Here's why:

* Sound, not Spelling: "An" is used before words that start with a vowel sound, even if the first letter is a consonant. For example: "an hour" (the "h" is silent, so it starts with a vowel sound).

* Smooth Flow: Using "an" creates a smoother transition between words, making the phrase easier to say. Imagine saying "a apple" vs. "an apple." The first sounds clunky and awkward.

Here are some examples:

* "a" before consonant sounds: a cat, a book, a dog

* "an" before vowel sounds: an apple, an umbrella, an hour

Let me know if you have any other questions!