ANUse the pronoun an before a word that begins with a vowel or the sound of a vowel. (Remember that the vowels are a, e, i, o, and u.)
It is essential not to miss the key word in the rule: the SOUND of a vowel.
In these examples, apple and octopus begin with vowels. The word hour, however, begins with the consonant H, but it sounds like the vowel O. Since we pronounce the H in hour like a vowel, use the article an.
AUse the pronoun a before a word that begins with a consonant letter or has the SOUND of a consonant.
In these examples, beach and towel begin with consonants. The word university begins with a vowel, but it has the sound of a consonant. The U in university sounds like a y in you-niversity. Therefore, use the article a.