Everyone already knows that Duolingo in the vacuum itself, is not a good tool to learn language. It’s a good spaced repetition tool though, especially the desktop version.

I’ve found that, I’ve learnt more with Duolingo by pairing it with my own grammar research, e.g. find the grammar book for the language I want to learn, then summarise / write down the rules lines by lines on my notebook

I’ve been doing it for a number of languages, and they have served me well. But only when I personally write them down line by line, the rules barely stick when I just read / refer to them like cheatsheets

It also helps a lot when you understand grammatical concepts like dative, accusative, ablative, instrumental cases, conjugation patterns, etc.

However this method is only good for learning to read and to listen, but not to speak, which in the end requires human interaction