When is 'an' not to be used with 'a e i o u' words?

[edmondng]

Dear Language/Grammar Advisor ...

I am aware 'an' is usually used to precede words with vowels, such as those beginning with a e i o u.

My question is, when is 'an' not used to precede words beginning with a e i o u?

For example, I am writing a document about 'a unified solution'. I know 'a' should precede the word 'unified' and not 'an'.

In this case, how should I explain this as a proper English usage? Are there resource or qualified documents that talks about this that I can use to explain?

Appreciate your advice. Thank you.


Regards
Tech Writer

[abaka]

When there's a [j] (iotization, semi-consonant i, y as in "you") sound in front of the vowel as it is pronounced, you write "a", non "an". It's a phonetic rule, not a spelling rule.

"a unified" = "eh, you-nified"

"an unacceptable" = "a nun acceptable".

Say it out loud. Do you see the difference?