When to use the indefinite article a and when to use the definite article the depends mostly on how specific you want to be. During a wedding ceremony the groom would say, "Give me the ring! The wedding ring!" because he must have a particular ring, while a carpenter would say, ‘Hand me a nail" because he doesn’t care which nail in the box he uses. Usually the bigger problem is not whether to use a definite or indefinite article, but whether to use an article at all.