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.
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.