(An) in front of the words that begin with a vowel sound but while I was reading

[Maw]

Hello everyone,
I know that you are busy so I will get straight to the point.We all know that we put
(an) in front of the words that begin with a vowel sound but while I was reading a story
I found (a) in front of (uniform) instead of (an)
why?
Thats it.
Thanks a lot.
Maw

[Vietanhpham_Winter]

Pay attention to the pronunciation. User, uniform, unicorn, university...u here is pronounced /j/ and j is just a semi vowel so it requires a.
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