Use an before a word that starts with a vowel sound. If it does not start with a vowel sound, use a.|
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 AnThere is sometimes confusion about whether to use an or a (particularly with abbreviations). 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.
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.
(LRS - Linear Recursive Sequence)
A TT race...
(TT - Tourist Trophy)
It would be a honour.
('honour' - starts with an o sound)
Send an US ambassador.
('US' - starts with a y sound)
She was involved in a RTA.
('RTA' - Road Traffic Accident)
Click on an or a:
WATCH OUT FOR THESE
Abbreviations that start with the consonants F, H, L, M, N, R, S and X attract an, because they start with vowel sounds.
An FRS representative will be present.
(FRS - Fellow of the Royal Society)
A LF transmitter was found in the
(LF - Low Frequency)
WATCH OUT FOR U
Abbreviations that start with the vowel U attract a, because U starts with the consonant sound y.
A US ship spotted a U-boat.
An UFO landed in 1967.
TREAT ACRONYMS LIKE WORDS NOT ABBREVIATIONS
An acronym is an abbreviation that is spoken like a word, e.g., BUPA, FOD, FEDEX. Therefore, as the first sound of FEDEX is f, use a and not an.
Tim worked in the air industry as a FOD inspector for a year.
(FOD - Foreign Object Damage)
Jack was a FEDEX courier.
AN HISTORICAL OR A HISTORICAL
Letters and sounds do not always correlate in English.
When pronouncing the words historic and historical, the accent falls on the second syllable, and many pronounce them as starting with a vowel. For those people, it is appropriate to use an before historic and historical. Therefore, you have a choice depending on what sounds best for you. There is a lot of leniency on this issue. If you're still unsure, opt for a historical and a historic as these remain preferable – especially in formal writing.