Use 'a' before a consonant SOUND and 'an' before a vowel sound. The letter is irrelevant.

The article exists to make the join between two words easy to say. That is a fact about sound, so a silent 'h' takes 'an' and a 'you' sound takes 'a', whatever the first letter looks like.

Except

'A historic' and 'an historic' are both accepted; the 'h' is weakly pronounced. The exam does not set this one.

an honest officer, a university, an MP, a European

sound decides every one of these

a honest officer, an university, a MP, an European

the spelling was consulted instead of the ear

Where it bites

Abbreviations, where the letter name is what is spoken: an MLA, an FIR, an SDM, a UPSC aspirant. Read the abbreviation aloud and the article follows.