Decay rate is insanely high
My problem with the Romanian course is that the decay rate is extremely high. Each and every day between 15 and 20 of my skills decay and are no longer gold. I have to spend at least an hour a day just to reiterate over them. There is no benefit from doing the same lessons again and again each and every day.
This insane decay rate is not something I'm used to in Duolingo, it takes the fun out of the learning process and is quite discouraging.
Does anybody know why is happens and how to deal with it?
It actually seems pretty normal to me. In Swedish and Spanish I would finish a skill and then a few days later I'd have to go back over it. This went really fast in the App (a strengthen takes me 3-5 minutes on my iPad), but on the site it's much slower. Two days later I'd be back at that skill because lesson 2 needed strengthening. By the third or fourth go-round on the skill it would mostly stay gold. At this point I just have to strengthen 1-4 times a day and I'm fine.
In Romanian I haven't really bothered as much with strengthening because it would just take forever on the site.
I can't answer your question, but I'll chime in and say that I've definitely noticed the same thing. I finished the tree about a month ago (would only move onto new lessons when I had all gold everything), and have done at least 50 XP (usually 100+) per day every day since then. I tried to keep everything gold but some days there would be 18 or 20 skills that needed refreshing and I couldn't do it all. At this point it's decreased a bit, it's only maybe 5 to 8 every day, but that still seems like a lot. I guess the good news is that it seems to be getting better.
My best guess is that the "strengthen skill" exercise for a lot of units does not cover all of the material in the unit. There are several skills where you need to go and re-do the parts of the original lesson in order to strengthen the skill. I think that the words not covered with the "strengthen skill" exercise are harder to keep strong, and hold back the entire lesson. That's just a theory, though.
Lessons don't weaken. Words do. When enough words drop below a certain strength you get a non-orange lesson. The weakest words get put in the Strengthening session.
Generally your first time going through a tree you'll do a lesson, 2-3 days later all the words in that lesson will decay, and you'll have to do a strengthening session that's basically a repeat of the lesson. I do two new lessons a day, so my first strengthening session will generally cover lessons 1-2, the very next day I have to strengthen 3-4, etc. I really ❤❤❤❤❤❤❤ hate 10-lesson skills. It tends to settle down after I've done the lesson the first time, and strengthened all words in it a couple times, because Duo cuts down the decay rate every time you learn something.
In both Spanish and Swedish this meant that I when I got to about 3/4 of the way down the tree I started get ridiculous amounts of decay, to the point I just said "screw it" and finished the tree before goldening. As I said above, I entered "screw it I'ma finish the tree first" mode on Romanian about a week-and-a-half ago because I really hate strengthen sessions on the website.
BTW; one way to game the system is to do the more advanced skills first. Every time you use a word in a strengthen session it strengthens, which means that if you're doing one of the bottom Romanian skills and "delfinul' appears you may strengthen the definite skill. If you just hit the barbells you will tend to get the top/most basic skills.