When should you use "a" and when should you use "an"?

In English there are two indefinite articles, a and an.

A is used when we refer to a word starting with a consonant sound, e.g. a doctor, a car, a marriage, a woman.

An is used when we refer to a word which starts with a vowel sound, e.g. an apple, an eagle, an octopus.

It is important to note that many words which start with a consonant letter may actually start with a vowel sound, and some words that start with a vowel letter may start with a consonant sound. For example, when a word starts with a silent h, it generally starts with a vowel sound, e.g. hour, heir, honest.

Words that start with a vowel letter but a consonant sound tend to be acronyms where the letters are pronounced individually, e.g. UFO: because U is pronounced as yew on its own, it takes the indefinite article a.