Japanese from English: Duolingo just doesn't prepare us for anything
Right after finishing hiragana, what comes is almost impossible to any learners from zero. Its ridiculous, its ludacris. Duolingo teaches us the alphabet and the next day give us Poe's poems to read, understand and discuss about.
From "Time 3" and after, you just can't go at all.
I agree with regard to the number system. It's written and spoken differently when using numbers with time, counting objects, and people. Duolingo doesn't offer any tips before introducing the different types. I guess their approach is sink or swim. I like to fully comprehend the basics before advancing and it's frustrating that the numbers are so complicated.
Different apps can have slightly different approaches.
Both LingoDeer and Memrise have free (Android) apps which can be used to supplement Duolingo.
If having kana problems, Michael Rowley has the Kana Pict-O-Graphix and Kanji Pict-O-Graphix books.
If grammar help is needed maybe using either: the Genki Textbooks and/or self-study oriented guides like Japanese the Manga Way by Wayne P. Lammers or Marc Bernabe's Japanese in Mangaland series can be helpful.
I had the same experience. The problem for me, I discovered, was that duolingo was not teaching any grammar rules. Please find Tae Kim's guide to Japanese for free online. Then, to get into the duolingo course, go question by question and look up the word at a dictionary like Jisho and figure the grammar from the book combined with the questions/answers.
Right now the course is not very good. I think there is something to be said for having tried to learn something using a method that does not work. But that is very hard to explain at the moment!
It's a difficult language to learn, but trust me that it becomes easier if you practice a lot. I agree there are a lot of ways where Duolingo can improve, but that mostly should be on explaining grammar, the use of particles, adjectives, verbs and some more kanji and less random use of kanji (words only half kanji, or in one lesson kanji and a few lessons later back into hiragana).