Before jumping into the Greek article we should first do a little review of the English articles. To begin with, what exactly is an
article grammatically speaking? In English we have two different types. They are the word "the" and the word "a" (or "an" when the following word begins with a vowel). The word "the" is called the definite article since it indicates that the noun it precedes is something that is known or is a definable entity. The word "a" (or an) is the indefinite article and does
not indicate a specific or known object. Let's look at an example to draw these concepts out.
1. The boy gave him
the apple.
definite article 2. The boy gave him
an apple.
indefinite article
In number one, we know that the apple being given is a known apple since "the" precedes the noun
apple. For instance, a possible expansion on this sentence could be:
A young boy walked down the road with a basket of apples. During this walk, he saw a poor man and took compassion him. He reached into the basket and grabbed the best apple he could find. The boy did not hesitate, he gave him
the apple.
When a definite article is used we should be able to answer the question. Which or what? That is, what apple did the boy give? In this case we can say "the apple the boy grabbed from the basket" and "the best apple the boy could find".
In the case of number two, we don't know any additional information about the apple. We could suspect to see it used in the following context.
The boy walked down the road with a basket of apples. He saw a poor man, took compassion on him, and gave him an apple.
In this case we can't say anything additional about the apple, thus the use of the word "an". Sure one could argue that the apple was taken from the basket but we don't know this. In fact, the boy could have been carrying both a basket of apples AND an apple in his hand. The use of "an" suggests that we don't know anything additional about the apple the boy gave.