Since user starts with a vowel shouldn't we use "an" ? I've seen many cases of using "a" .
"An" goes before all words that begin with vowels:
- An egg
With two exceptions:
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:
- a union
- a united front
- a unicorn
- a used napkin
- a U.S. ship
- a one-legged man
It's a because the first sound of user is not a vowel, but the consonant /j/.
‘Vowel’ and ‘consonant’ describe letters that represent vowel and consonant sounds, but they also describe the sounds themselves. A vowel is a sound made from the throat without interruption by the other vocal organs. A consonant is a sound blocked or restricted by audible friction. The initial sound of ‘user’, /j/, is interrupted by the position of the soft palate and the tongue. It is convenient to group it with the other consonants, but, because its place and manner of articulation are a little different from them, it is also known as a semi-vowel.