Articles: A versus An । English Grammar Rules । আর্টিকেল রুলস

The indefinite articles
The selection of article is actually based upon the phonetic (sound) quality of the first letter in a word, not on the orthographic (written) representation of the letter. If the first letter makes a vowel-type sound, you use “an”; if the first letter would make a consonant-type sound, you use “a.” However, you may follow these basic rules when deciding to use “a” or “an,” remembering that there are some exceptions to the rules.
“A” goes before words that begin with consonants.
•    a cat
•    a dog
•    a purple onion
•    a buffalo
•    a big apple
“An” goes before words that begin with vowels:
•    an apricot
•    an egg
•    an Indian
•    an orbit
•    an uprising
Exceptions
Use “an” before unsounded “h.” Because the “h” hasn’t any phonetic representation and has no audible sound, the sound that follows the article is a vowel; consequently, “an” is used.
•    an honorable peace
•    an honest error
When “u” makes the same sound as the “y” in “you,” or “o” makes the same sound as “w” in “won,” then a is used. The word-initial “y” sound (“unicorn”) is actually a glide [j] phonetically, which has consonantal properties; consequently, it is treated as a consonant, requiring “a.”
•    a union
•    a united front
•    a unicorn
•    a used napkin
•    a U.S. ship
•    a one-legged man