"degli" is the partitive article (articolo partitivo) in this sentence. It means an indefinite part of a whole, or "some", however it is not translated in this case.
If someone could please explain in further detail why the partitive article is used in this sentence or give a summary of when it should be used that would be great.
Perplexing! Why not "the teenagers become the adults". Now, in English, I can imagine "teenagers become adults", "The teenagers become adults", and "the teenagers become the adults" all being used by a native speaker in various contexts. (duolinguo does not provide contexts) So the question is, how would you say "The teenagers become the adults", in Italian.
See f. formica above; he is a native speaker. As he explains, tacking in the PARTIVE ARTICLE "degli" instead of just "gli", changes it to 'become adults" instead of "become THE adults".
Gli adolescenti diventano GLI adulti = The adolescents become THE adults.
Gli adolescenti diventano DEGLI adulti = The adolescents become (some) adults or, just generally, adults.
The partive article usually changes the sense of the meaning to "some" or "any" of whatever; but in this case, in English one wouldn't say "some adults" so it translates just "adults".
Here are some websites with explanations on partive articles:
Well, first off, Tom, I don't know what's "bad" about my English depending upon the context of the sentence, which is always absent... But all I'm saying is that the simplest way to say "The teenagers become adults," is most likely by doing away with a misleading article, and it's probably perfectly legitimate to do so in spoken Italian, given the language's several ways to say any one thing. Handing me "degli," my mind moves first to consider "some." That not sounding correct, it moves to "the." At the end of the day, if the composers of the sentence would like to have expressed simply "adults" sans article - as can happen throughout the language, apparently more or less at random - why didn't they just say it. The method of teaching is confusing and counter-productive. And that's the point I've been attempting to make for several years. Even my native speaking friends find such sentences as these, stuffed with unnecessary grammar, ridiculous.
I sincerely apologize, Keith, if my phrasing offended you but I still believe that the sentences you proposed in your first comment do not sound natural. At least I would never use the second definite article before “adults”. I do believe the ‘English to Italian translation’ form of this exercise should accept the version without “degli” which I reported and commented about above. However, this minor persisting error does not warrant your harsh criticism of Duolingo. Afterall, one of the pillars of Duo is crowdsourcing. You wrote: “…which means that "degli" in the Italian translation means nothing. It's just...there... Come on, Just stop!!!” – if you get the Italian sentence and are supposed to translate it into English, then you just can’t do so word-for-word (“degli” won’t have an English equivalent). That’s how the language works. Deal with it. Another thing you are missing is that there is no one perfect way to teach a language. You can do it by studying grammar, dissecting sentences and understanding how the logic of the language works. Or you can just listen, repeat, translate and learn from mistakes until you develop a feeling for the language. Duo aims more for the latter. I admit that Duo is not perfect, and a lot of time lack of context is a problem. But in general, the strategy is valid and works for many people. If you really want to learn a language efficiently, you have to complement Duolingo with other sources. Like in this case, instead of complaining that Duo threw “degli” at you, it should spark your curiosity to find out more about this feature of Italian. And whether you choose to look up the grammar to fully understand it or multiple examples to get a feeling for its use is totally up to you and your learning preferences. I believe you could have easily inquired about this topic constructively here in the forum instead of calling it “unnecessary grammar” and “ridiculous”. That’s just not fair from you.