You use a or an when you are referring to someone or something for the first time or when people may not know which particular person or thing you are talking about.

You use a or an when you are referring to any person or thing of a particular type and do not want to be specific.

You use a or an in front of an uncount noun when that noun follows an adjective, or when the noun is followed by words that describe it more fully.

You use a or an in front of a mass noun when you want to refer to a single type or make of something.

You use a in quantifiers such as a lot, a little, and a bit.

You use a or an to refer to someone or something as a typical member of a group, class, or type.

You use a or an in front of the names of days, months, or festivals when you are referring to one particular instance of that day, month, or festival.

You use a or an when you are saying what someone is or what job they have.

You use a or an in front of the names of people as a way of indicating that you do not know them or anything about them and you are saying their name for the first time.

You use a or an in front of the names of people when you want to refer to someone else who has the same qualities or character as the person named.

You use a or an in front of a surname when you want to refer to someone who belongs to the family with that surname.

You use a or an in front of the names of artists to refer to one individual painting or sculpture created by them.

You use a or an instead of the number 'one', especially with words of measurement such as ' hundred', 'hour', and 'metre', and with fractions such as 'half', 'quarter', and 'third'.

You use a or an in expressions such as eight hours a day to express a rate or ratio.

A- is added to the beginning of some adjectives in order to form adjectives that describe someone or something that does not have the feature or quality indicated by the original word.