are used to refer to a particular member of a group. It may be something that the speaker has already mentioned, or it may be something uniquely specified.

For example, Sentence 1 uses the definite article and thus expresses a request for a particular book. In contrast, Sentence 2 uses an indefinite article and thus conveys that the speaker would be satisfied with any book.

# Give me the book.
# Give me a book.

The definite article can also be used in English to indicate a specific class among other classes:
:

1.Previous
3.Next