The script of our English language consists of 26 alphabets out of which five are vowels (a, e, i, o and u) and remaining 21 are consonants.
Any word starting with a vowel such be preceded by “an” for example:
An Aeroplan, An Evening, An Owl, An Elephant, An Ant etc.
But there are some exceptions in this rule which states that there are even some words which starts with a consonant but in preceded by “an”, this happens when the consonant is silent and as a result the word is pronounced beginning with a vowel for example:
An hour, An honor, An heir etc.
Any word that begins with a consonant is being preceded by “a” for example:
A boy, A girl, A mango, A cup, A shirt, A glass etc.