1.The articles 'A' and 'An' are indefinite articles. They are used before a singular countable common nouns.
A feather is lighter than an elephant.
The article 'a' is used to express quantity.
A little sugar, A large quantity, A huge amount.
2.We use “A” before nouns or adjectives which begin with a consonant or when U sounds like a Y, or O sounds like a W.
A one-eyed man
3.We use an before words which begin with a vowel sound or a silent “H” sound.
An antique necklace