A or an?

[stewart]

I'm occasionally confused about when to use 'a' and when to use 'an' - is it, for example, 'a drugs agency' or 'an drugs agency'? 'A' drugs agency sounds better, but is it 'an' because it's 'an agency'? Could you help me please.

[Casiopea]

Use "an" if the word (it sits directly in front of) begins with a vowel sound, and use "a" if the word begins with a consonant. Like this,

an apple, but a big apple.

an agency, but a drug agency.

Watch out! Some letters look like vowels but sound like consonants, like, USA,

a USA agency <Note, USA is pronounced, [ju:] or [yu:]
a union bank <Note, union, [ju:] or [yu:]
a UFO

The letter "u" in those words begins with a consonantal sound, [j] / [y], so use "a", not "an". Not all u-initial words are pronounced that way, though; e.g., umbrella, ulcer, umpire, etc. Those words take "an".

Remember, the key to using "an" and "a" is sound. It's in your ears, not in your eye-sight.