Greek does not use the indefinite article.

English uses the indefinite article for the singular (a, an) but does not use it for the plural.

Any noun with an article in Greek is definite.

The fact that there is no distinct sign for the indefinite noun need not lead to confusion. Observe that in English, for example, we have both positive and negative numbers, but we generally use a sign only for the negative numbers. If a number does not have this negative sign, we understand the number to be positive. So with the noun in Greek: if it does not have the definite sign (i.e., the definite article), we understand that noun to be indefinite.