There are very specific rules regarding a/an.
It's important to note (like someone already mentioned) that what is important here is pronunciation, NOT spelling.
In most cases they overlap though so it's not that difficult to guess if you don't know how to pronounce a word.
We put 'an' when the first sound of the following
word is a vowel (a, e, i, o, u). Again, think pronunciation!
at (sound /k/)
ice cat (sound /n/)
gly cat (sound /a/, well the exact phonetic symbol is different, but for simplicity let's assume it's /a/ like 'u
nion cat (here, 'u' is pronounced like 'you' - the sound is /j/)
lephant (sound /e/)
gly elephant (sound /a/)
ice elephant (sound /n/)
mmature elephant (sound /i/)
ery immature elephant (sound /v/)
xtremely immature elephant (sound /i/)
The only problem here might be words starting with 'u'. Sometimes the first sound is /a/ like in 'u
gly', and sometimes /j/ as in 'y
/a/ - ugly, unimportant, umbrella, etc.
/j/ - university, union, united, and probably some more...
As long as you know how to pronounce a word, it should be clear what to put in front of it.
btw, People whose first language is not English might find learning English phonetic symbols extremely useful:
If you know them, you can look up a word in a (good) dictionary and you'll know exactly how to pronounce a word.
It might not mean that automatically you'll pronounce them like native speakers do, but at least you'll have the exact idea how they should
Coming back to a/an, As someone already mentioned, the only confusing area MIGHT be words starting with 'h' in spelling. The pronunciation varies
depending on regional accents. For example, in some areas of the world they might pronounce the first 'h' in 'honour' (as in 'home', 'hand', hat, etc),
whereas people living in other parts might tend to skip the 'h' eg. hour = our.
an hour ago (silent 'h', so the first sound is /au/
a horror film (here 'h' is not silent)
I don't think though that it's something that people should worry about. There are not many words like that. And besides you can always stick to the
standard pronunciation. After all, the reason why standards exist is to avoid confusion and misunderstanding.
btw, It wasn't that one day some grammar freak woke up and said: What can I mess up today?! I know. Let's introduce the distinction between 'a' and 'an'!!! Yeah, That's it!!! That will teach them!!!
When you think of it, the distinction is natural and justified:
When we speak, we usually speak quickly. Particular words blend in a sentence.
Try to read quickly: I've seen aelephant (a elephant), I'd like aapple (a apple), I'm aintrovert (a introvert), etc.
It sounds much more natural to put 'something' between the two neighbouring vowels: a(n)elephant, a(n)apple, a(n)ugly cat, etc.
I hope it makes sense