When to use the indefinite article (a/an)

• You can only use a or an with countable nouns that are singular.
• You must use a before consonant sounds (a university) and an before vowel sounds (an hour). 
• You use a/an when you are not referring to a specific person or thing and when talking about jobs or roles (He is a teacher). 
• You use a/an + noun + of to talk about a unit of something uncountable (a piece of news).