This is because of the vowel of the following word...in english grammer we've learned if some word starts with vowel we must put "an" before this one but if do not so we just use "a". For example: I did eat a apple. THIS IS INCORRECT AT ALL BECAUSE OF THE "A" PRECEDING THE WORD "APPLE". the correct structure of this sentence would be... I did eat AN APPLE. Just another example: I can't wait to eat A CANDY. IN THIS CASE THE SENTENCE IS CORRECT. Capiche?

Though the word 'union' is spelled with a vowel as its first letter, it's pronounced as if it starts with a 'y', which counts as a consonant in this case. That's why would you would also say "a european" or "a university".

Yeah, it is help me much, thanks for the support.

In English phonology, vowels have 15-22 vowel sounds depending on your accent. In my Australian accent I use 22 vowel sounds, however American accents typically only use 15 (varies with state). This is unlike a language like Spanish that only has 5 vowel sounds. In the case of the words 'union' or 'unite', the 'u' is phonetically spelled as the dipthong 'yoo'. Grammatically speaking 'an union' may actually be correct, I'm unsure. But phonologically it sounds wrong to say 'an u' because we HEAR it as 'an yoo' and sounds as if it starts with a consonant. Hope that helps. Edit: Great question by the way Robert, a lot of native speakers of languages with fewer vowel sounds are confused by this. I recommend reading about English Phonology to improve your understanding.

Thanks a lot Sarah... have a nice night