"a" is used before a word with a consonant sound in the beginning and "an" is used in an instance where a word begins with a vowel sound. Don't let any British people tell you otherwise: the change doesn't apply to whether it starts with a vowel or not, it applies if it starts with a vowel sound or not. Vowel sounds are made by a,e,i,o,and u, though not always. A dog, a horse, a tree, a universe, a house, a carpet an apple, an agent, an operation, an idiot, an eagle

"A" is written before nouns starting with a consonant. For example, "a cat" or "a boat." "An" is written before nouns starting with a vowel (a, e, i, o, u). For example, "an apple" or "an elephant." I hope this answers your question.

As the others have said, an a is used before a consonant and an is used before a vowel. But there is one exception. If a word beginning with h but sounds like it begins with a vowel is used, then use an. For example: it is an honour /honor to meet you. Honour here is pronounced as if there is no 'h' like 'onour '. Same for the word 'hour' which is pronounced 'our'.