"A" or "An"?
Use 'an' before a word that starts with a vowel sound. If it does not start with a vowel sound, use 'a'. For example:
A man
An elephant
But, look at this:
A house
An hour
The key word here is sound. It is not a question of whether the word starts with a vowel. It is a question of whether it starts with a vowel sound.
Using A and An
There is sometimes confusion about whether to use an or a, particularly with abbreviations. (The words an and a are known as articles.) The sound of a word's first letter determines which to use. If the word starts with a vowel sound, you should use an. If it starts with a consonant sound, you should use a. Examples:
Buy a house in an hour.
(Although house and hour start with the same three letters (hou), one attracts a and the other an.)
An unknown goblin killed a unicorn.
(Although unknown and unicorn start with the same two letters (un), one attracts 'an' and the other 'a'. Remember, it is about the sound of the first letter.)
An LRS... (LRS = Linear Recursive Sequence)
A TT race... (TT = Tourist Trophy)
It would be an honour. (The word honour starts with an o sound.)
Send a US ambassador. (The abbreviation US starts with a y sound.)
She was involved in a RTA. (RTA = Road Traffic Accident) (The abbreviation 'RTA' starts with an 'a' sound (i.e., ar-tee-ay.)