I know the a/an rule, but as I meet both in my readings, I tried to google it, and I find both, with not site or dictionaries telling us we should use the one or the other one.
My question is: are they both common and accepted? Are they exceptions?
My favorite example is
An uninformed man
A uniformed man (a you-knee-formed man).
You use an if the next word sounds like it starts with a vowel, a if the next word sounds like it starts with a consonant.
There are cases where there is no universal agreement about pronunciation. Some people say "a hotel" (a hoe-tell), some say "an hotel" (an oh-tell).
When abbreviations are written down, some people will read them aloud as the letters in the abbreviation, and some will read them aloud as words. NDA is read as "en-dee-aye" or "non-disclosure agreement"; you use an or a depending on how you read it.