"He is a good boy."
Translation:Er ist ein guter Junge.
To put it simply, when the article does not give enough information to determine gender and case you must add the appropriate ending to the attributive adjective.
In this example, Junge is in the nominative case. Since both Masculine and Neuter Nouns take the indefinite article "ein" in the nominative, a case ending is required on the attributive adjective to help distinguish the gender and case of the noun. (Here are two examples to illustrate: "Er ist ein guter Junge." Masculine in Nominative case, and "Es ist ein gutes Mädchen." Neuter in Nominative case. This is different than with definite articles in the Nominative case, since they clearly differentiate the genders of the noun and as result all take definite article + "gute".)
(Why this is so I cannot say, as I was simply told that this is the way that it is done in German. My guess is that at some point it mattered more when word order was less standard and as a result the cases were more critical to the understanding of sentences.)
It is in the nominative case, Junge = masculine, ein = indefinite article. All of this together should help you find your answer in this mixed inflection chart: http://en.wikipedia.org/wiki/German_declension#Mixed_inflection.5B6.5D
That's why I like this program/website (duolingo); Full sentences/complete concepts are far more important than word-specific word-for-word translations.
That's why I think the lists of words other languages don't have a "word" for are funny. The real question should be, "Do they have a way to express the concept?"