Where a word begins with a vowel, (a, e, i, o, u), use An. Here are a few examples: An onion, an umbrella, an egg, an apartment. However, there are some words that start with vowels that are not preceded by An. The reason for this is that these words don't sound as if they start with vowels – they sound as if they start with consonants. Where a word begins with a ‘yoo’ sound, you need to use A instead of An. Here are a few examples: A unique painting, a unit of currency, a European country.
Where a word begins with a consonant, use A. Here are a few examples: A carrot, a table, a mountain. There are some words that start with consonants, however, that are not preceded by A because these words don't sound like they start with consonants – they sound like they start with vowels. Where a word begins with a silent 'h', use An instead of A. Here are two examples: An honest mistake, an hour.
What about abbreviations? Use the way the first letter sounds to guide you. A first letter that sounds like a vowel is preceded by An. A first letter that sounds like a consonant is preceded by A. (The 'yoo' sound is treated the same way as a consonant.) Here are five examples: 1. An AC circuit ('ay' vowel sound) 2. An FM radio ('eh' vowel sound) 3. An HIV medication ('ay' vowel sound) 4. A PG rating ('pee' consonant sound) 5. A UN delegate (‘yoo’ consonant sound)