Treccani is useful for learning Italian
If you register on Trecanni, the above free course is available. It can add the context of real situations and listening, so that you can start to progress what you have learnt with Duolingo to fluency. I found it from Lucrezia, who gives YouTube videos on Italian.
On Duolingo, I am working on Level 5 on most skills and on Level 4 for about 10 skills. I nearly have a golden tree.
If I had written it, there would be just as many errors. Duolingo has errors. You could email Treccani suggesting an error-reporting system.
Yeah, but this course is from an Italian encyclopedia site, probably made by native Italian speakers... You'd expect it to be pretty high quality. But oh well, at least it's free.
I really like how everything is in Italian though; it's nice to read about the grammar rules without having to compare them to any other language.