[Grammar] a or an

[elenah]

Hello,
Can you tell me which article is right?
I was taught that you should put 'an' before a word that begins with a vowel letter (never mind exceptions). But someone told me recently that I should put 'a' before 'employer', is that true?
Which is correct: an employer or a employer?

Thank you in advance

 

[bubbha]

Also:
an MBA student
an hour

And yes, "an employer" is correct. You may hear some people use "a" before a vowel sound, but it's non-standard.