used before countable or singular nouns referring to people or things that have not already been mentioned, used before uncountable nouns when these have an adjective in front of them, or phrase following them, any; every, used to show that somebody/something is a member of a group or profession, used in front of two nouns that are seen as a single unit, used instead of one before some numbers, used when talking about prices, quantities and rates, a person like somebody, used before somebody's name to show that the speaker does not know the person, used before the names of days of the week to talk about one particular day, Used when referring to someone or something for the first time in , Used with units of measurement to mean one such unit, One single; any