It's not the plural in this case, it is because of the dative case. Ah, I am trying to think of any other masculine nown where you have to do that, but I can't come up with any. "Junge" is an exception because of the "e" at the end I think (masculine nowns don't usually end in an "e"). Maybe someone else can explain more exactly why this is done, but it is necessary.
'Junge' belongs to the class of weak nouns which are inflected differently from the normal strong nouns. There is a third inflection class, namely nominalized adjectives that get inflected exactly like the corresponding adjectives. See here: http://german.about.com/od/grammar/a/dernouns.htm Note that german.about.com lists 'der Alte' as a weak noun which is not totally accurate. 'der Junge' is a weak noun but 'der Alte' is a nominalized adjective. You can see the difference in inflection if you change the article: It's 'der Junge', 'der Alte', 'ein Junge' but 'ein Alter'.
Ok so I tried reading that link you posted by I only became more puzzled. 'Jungen' usually refers to 'boys' but in this sentence it referes to 'boy', so then how exactly would one know when Jungen in a sentence like this would refer to plural boy, or in other words, boys? Would the sentence structure/format change or something?
- singular declension: der Junge, des Jungen, dem Jungen, den Jungen
- plural declension: die Jungen, der Jungen, den Jungen, die Jungen
You have to infer case and number from cues in the sentence, e.g. verb conjugation, prepositions, etc. In our example, the article 'dem' tells you that it's singular dative.
I really feel like some of these could use a better introduction or explanation. I like that duolingo just tosses things at you with a trial by fire kind of thing, but for German you really really need to introduce cases and conjugations at the beginning of learning, even if it's just tossed at you.
As for this particular instance, I think a section specifically on dative/possessives would be a good idea. For those who might be struggling on these translations, think of it this way: In English it's - The boy's shoes. In German it's - The shoes of the boy.