A, an, and the are called articles. The definite article the is used to refer to a specific noun, while the indefinite articles a and an are used to refer to non-specific nouns. Consider the following examples.
1.) Please give me the notebook.
2.) Please give me a notebook.
In the first sentence, a specific notebook is referred to. It is assumed that both the speaker and the listener know which notebook is referred to. On the other hand, in the second sentence, the speaker asks for a notebook, any notebook will do.
Rules in Using A and An
A and an are both used to refer to non-specific nouns but there are rules that you must remember to use them. » Read more