WOW!. thank you so much for this link. I have been lost in duolingo for weeks. It seems like the horse is in front of the cart [so to speak] I can't unlock things it seems like there is no order to learning. Within minutes reading from this link and learning basic grammar rules I felt much more grounded in understanding and going back to duolingo, well that is, if I go back. thx again!
What I do (since you asked) is ignore the fact that the next lesson has become unlocked when that happens, and I stay with lesson I am on till I have mastered it. I want to see one hundred percent accuracy. That means, all three hearts still standing when a lesson is done. This earns one a lingot, by the way. But once is not enough. I continue with the lesson until I have finished it three times with no errors. This means I will be doing the lessons many dozens of times, over and over, and I will be seeing new questions and new words, too, which get added in. More challenging questions,too, which only the students working the lessons like I do ever see. Then after a perfect score times three I allow myself to go to the next lesson. And only then. If a person rushes through the lessons seeing how fast they can go they will be only using short term memory and that person will soon forget everything. It's the way short term memory works and is supposed to. Doing it my way helps to establish everything in long term memory. Oh, after I complete ten lessons I start all over again from the begining. Again and again.. For me, this is no game. It is serious hard work. And I intend to actually learn the language.
It has to do with the rules for accents. The natural stress falls on the second to last vowel unless it has a diacritical mark (e.g. esta vs. está). In the case of joven, the natural accent falls on the o. In the plural, we want the accent to stay on the o, but now it is the third to last, not the second to last, vowel. Hence the need for an accent.