1- Indefinite article (a/an)

If the noun is singular and countable, and this is the first time you have mentioned it, then you will usually need the indefinite article: I bought a book – we do not know which book. There is a bird outside – we do not know anything about the bird. Measurements and rates also take the indefinite article: Three times a week If the noun starts with a vowel sound, then the article an is used: an ear, an uncle, an hour If the noun starts with a consonant sound, then the article a is used: a school, a university