Why shouldn't we say "an user"? [duplicate]

Solution 1:

The word uniform begins with a palatal approximant /j/. The palatal approximant is a consonantal sound, even though the letter itself is a vowel. Since we use the pronunciation of the word following the article to determine whether we use "a" or "an", and because it is pronounced starting with a consonant, we use "a". Thus, the original sentence is correct.

Just for the sake of completeness, the letter "u" can also have a vowel sound, such as in the word "umbrella", which begins with an open-mid back unrounded vowel /ʌ/.

Solution 2:

The rule of placing an before a vowel is actually to place an before a vowel sound. Therefore, because uniform is pronounced with a "y" sound (/ˈjuːnɪfɔːm/), which is not a vowel sound, it takes an a before it.