A或An

Question: Why do we say a union and not an union?  Why do we say an MBA when MBA starts with a consonant? 

Grammar: Indefinite Article 

The general rule for indefinite articles (a or an) is:

• If the next word starts with a vowel (a, e, i, o, u) then use an.
•  In other cases, use a.

However, the more accurate rule is:

• If the next word starts with a vowel sound, use an. 

So for example, MBA doesn’t start with the /m/ sound.  It starts with /em/, and /e/ is a vowel sound.  Therefore we write an MBA.

The word union may start with U, but when you pronounce it, it actually starts with a Y sound, or /j/.  This Y sound is not considered a vowel sound, and so you should say a union.