Hey, I remembered a book on super memory which I have recently read. And here, I am applying what I have just learnt.
The rule to remember is:
- Use 'a' when the word begins with a consonant sound (including 'u' which is pronounced as 'yoo'). E.gs: 'a bad service experience', 'a seller', 'a buyer', a holiday', 'a union', 'a university', etc.
- Use 'an' when the word begins with a vowel sound or silent 'h'. Egs: 'an hour', 'an apple', etc.
- The same rule applies to groups of letters and initials: 'a' before B, C, D, G, J, K, P, Q, T, U, V, W, Y, Z; and 'an' before A, E, F, H, I, L, M, N, O, R, S, X. [Memory Boaster: A Bride Weds and Dances Very Well To the Zoo, With a Guy Called Jack. A King, Queen and Prince Yakked about the Union. An Elephant Feels Hot In London. Monkeys, Not Xenophobic Octopus, Rule Ship.]
- Words beginning with 'h' and an unstressed syllable formerly took 'an' but 'a' is now usually preferred. Therefore, 'a hotel', 'a historian'.