Bunpo for Android is a Japanese grammar study app that was previously released on iOS. Covering JLPT N5 to N1, it's seriously packed with content—and the polished illustrations and UI are a nice touch too!
When you launch the app, choose a JLPT level or basic kana lesson (the app also teaches hiragana/katakana if you're a beginner). You'll see a collection of cards containing grammar lessons you can study and review in any order.
First, as you swipe through cards, you're taught each grammar point with explanations and example sentences. If you're having trouble reading the sentences, example audio is available (courtesy of an okay-sounding robot voice) as well as optional English translations. After that, you're immediately quizzed with a series of multiple-choice and choose-the-word-order questions.
For an app, Bunpo's explanations + exercises are pretty long, which helps to make up for the lack of variety and the multiple-choice-iness of the review tests. The Bunpo download page makes claim to 1,700 example sentences and 8,000 quiz questions total, all of which you can access offline.
Bunpo has a few shortcomings: wonky formatting errors sometimes cut off the ends of sentences or squish furigana together. And speaking of furigana, there is an on/off toggle, but it's not clearly marked (hint: you have to click the monkey in the top-right corner).
Still, Bunpo offers a ton of quality content for a no-cost, ad-free app. It won't replace the kind of lengthy and detailed grammar explanations you'll find in textbooks like Genki, but it's great for introducing new concepts quickly and giving you grammar practice on the go. This is one to keep on your home screen.