I've seen quality drops of Duolingo, ever since their ... IPO, sadly.

Anyway, here's some ways you can milk the rest of the Duolingo before completely abandoning it.

• Use the web version, and type in all the answers if it's possible. Selecting words are good for introducing new words (and reminder in case you forgot), but by typing it on your own, it's faster to commit into memory.
• Use classroom mode to get unlimited hearts, create your own classroom and invite yourself in. I assume that Duolnigo will probably eventually stop this loophole
• Use search engine to search for the sentences you're unsure of. No, don't use machine translation, but search on the internet, and see if the sentence ever being used by the sites (news, academic, or personal homepage) using the target language.

I sadly still don't know what other comparable free alternatives to Duolingo. Anki is great, but it's largely flashcard for words, not sentences (unless you want to create your own deck). The others require subscription fee.

Other methods? Search for pdf of language grammar files, there are a lot out there. Some are godawful to read, especially those 'Comprehensive Grammar Guide' books. Some are amazing, e.g. Tae Kim's Guide to Japanese.