"an usable" is a common category of mistake from non native English speakers that I see frequently and I don't judge for it.
Often I find myself trying to politely explain the rule. The "n" appears when there is a phonetic vowel next, not necessarily a written one. The first letter of usable is a vowel in spelling but phonetically it starts with a consonant, the international phonetic alphabet for it is /ju/, starting with the consonant [j].
Similar things happen with abbreviations. You get prescribed "an SSRI" and not "a SSRI" because "S" there is pronounced "es", starting with a vowel.
Strangely enough I find when I over explain like this about this specific topic, people thank me. Maybe since I'm way off topic I'll get crap for it here.
Often I find myself trying to politely explain the rule. The "n" appears when there is a phonetic vowel next, not necessarily a written one. The first letter of usable is a vowel in spelling but phonetically it starts with a consonant, the international phonetic alphabet for it is /ju/, starting with the consonant [j].
Similar things happen with abbreviations. You get prescribed "an SSRI" and not "a SSRI" because "S" there is pronounced "es", starting with a vowel.
Strangely enough I find when I over explain like this about this specific topic, people thank me. Maybe since I'm way off topic I'll get crap for it here.