Here's how it works:
* "A" is used before nouns that begin with a consonant sound:
* Example: "a cat," "a book," "a university" (even though "university" starts with a vowel, the "u" sound is like a consonant)
* "An" is used before nouns that begin with a vowel sound:
* Example: "an apple," "an hour," "an umbrella"
The key is the sound, not the letter. This means that even though some words start with vowels, they might still take "a" if the initial sound is like a consonant.
Here are some examples of words that start with vowels but take "a":
* "a European" (because "European" begins with a "y" sound, which is a consonant)
* "a one-way street" (because "one" starts with a "w" sound, which is a consonant)
* "a UFO" (because "UFO" starts with a "y" sound, which is a consonant)
Let me know if you have any more questions!