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!

a cat (sound /k/)
a nice cat (sound /n/)
an ugly cat (sound /a/, well the exact phonetic symbol is different, but for simplicity let's assume it's /a/ like 'under')
but:
a union cat (here, 'u' is pronounced like 'you' - the sound is /j/)



an elephant (sound /e/)
an ugly elephant (sound /a/)
a nice elephant (sound /n/)
an immature elephant (sound /i/)
a very immature elephant (sound /v/)
an extremely immature elephant (sound /i/)

The only problem here might be words starting with 'u'. Sometimes the first sound is /a/ like in 'ugly', and sometimes /j/ as in 'yes', 'you', 'young'.

/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:
http://www.wordreference.com/es/phonetic_symbols.htm
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 be pronounced.


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'veseena(n)elephant, etc.

I hope it makes sense

Note to the whole of the United States of America: please be aware that the word herb starts with an 'H' and therefore follows this rule. Therefore it is 'a herb' NOT 'an erb'. That is all.

Thanks Sycamorex, a very succinct and useful pronunciation guide.

In general (as opposed to "in General" ) most people of any country are forgiving of people who don't speak a language at least trying. This is why people who post here and mangle English get a much easier ride than those who either post in their native language or use textspeak to try to bypass the language barrier. Same applies with the spoken language.

what did u mean here w.r.t your example 'unfriendly'.

The decision on whether to use 'an' or 'a' is based on the pronunciation of the first letter of the word. In the case of words beginning with 'u' one would use 'a' when the u is long (as in 'a uniform') or 'an' when the u is short (as in 'an unnecessary'). Any letters that follow are disregarded when choosing 'a' or 'an'.

'A' Words:
uniform, union, useless

'An' Words:
ugly, unnecessary, unfriendly

I have understood it now but it is difficult for me to follow.

P.S.
Did you read your last visitor msg ? Perhaps you should block it !

Does this sentence make sense ?
or it should be

I'd use "You don't appear so frightening now" if the intended meaning is "less frightening".

The meaning of "You don't appear to be frightening now" is different, meaning "not frightening". It could be shortened to "You don't appear frightening now" without loss of clarity and with a gains in economy, directness and naturalness.

That makes sense.

Where have you been Charles, not seeing enough posts from you now-a-days

Anisha
It was brainL who suggested charging you money in those earlier posts. Part of his plan to make me seem to be the evil twin.

Where from do you get all these original ideas Briany ? It is difficult to avoid smiling on every post of yours.

Hi Anisha, thanks for asking. I'm travelling so intermittently on line and busy.

What evidence do you have to suggest there are only two?

There could be even more that I'm not aware of.

You shouldn't have said that!