Tae Kim's Guide to Learning Japanese is the OG free grammar resource on the Internet. If you've been researching the easiest and cheapest way to study Japanese grammar online, you have absolutely seen this name come up dozens of times.
But did you know there is an app?
Yes, Tae Kim's app has been around, but it just got its first update in five years, which means it looks much better for those of us with phones newer than the iPhone 5 (yikes!). If you've used this app before, visually, it will look familiar, but the styling, speed, and ease of use have all improved.
Lessons are broken up into ordered categories, like any textbook, and they include written explanations, embedded videos, charts, examples, audio, and practice exercises. Japanese words used inline have dotted lines, allowing you to tap them to reveal their English meaning and reading in kana, instead of using furigana. And of course the content itself is well-written, thorough, and goes all the way from super-beginner to more advanced topics.
The app also lets you bookmark chapters, choose from light or dark mode, and customize your font size. You also have the ability to search the entire site in-app, which is extremely handy.
Overall, this is a very welcome update that will hopefully let you bring your favorite free Japanese grammar resource with you on the go!