Functions of the Indefinite Article

Classifying	Generic	Numeric	Aspective
The article shows that the noun belongs to a certain class of things.	The article shows a typical member of a certain class of things.	The article shows its original meaning of the numeral ‘one’.	The article shows a special aspect of the notion expressed by an uncountable abstract noun.
This flower is a rose. They bought her a book as a present.	A rose is a flower. (= every rose is a flower)	Henry gave Lydia a big red rose. An apple a day keeps the doctor away.	With that rose he offered her an unusual love. It was an elation that seemed to give her wings.

Functions of the Definite Article

Specifying (individualizing) function	Generic function
The article shows a particular object that is singled out from the whole class of these objects.	The article shows that the noun means the whole class of things it denotes.
The book I need is on the table. He was eating the bread that the servant had brought him the day before.	The book is a great source of knowledge. Nobody knows when the violin was first made.

Don’t confuse the articles in the generic function

a	the
The article shows a typical member of a certain class of things.	The article shows that the noun means the whole class of things it denotes.
A writer is someone who writes books, stories etc, especially as a job. (gives a definition of any writer’s job)	Should the writer be impartial while writing his books? – (implies the idea of the writer’s responsibility)