I had been struggling with Japanese grammar for years, even though I had been taking weekly private lessons, and even after reading the very decent Tae Kim’s Guide to Learning Japanese, until I finally found the Organic Japanese with Cure Dolly YouTube channel, particularly the Japanese from scratch: the game-changing course in organic Japanese playlist.
It honestly feels like this is the one and only source that really groks Japanese. Everything is put logically, every sentence deconstructed, each topic given several examples, each explanation given its reasoning. She makes a point to demonstrate that there are very few exceptions and “random” things in Japanese, always going deep into why something works how it works. There are also a few miniseries (two so far), my favorites, where she reads stories, not translating them but deeply explaining line by line.
Now, I feel that I need to be upfront about some downsides. Her choices of aesthetics are… peculiar, to say the least. Her use of SFX is particularly upsetting, sometimes almost blowing my earphones. Her Japanese pronunciation also has a very pronounced accent, and it’s sometimes hard to understand her because of the voice modulation. This is a minor problem though since she has subtitles for all of her videos.
I made a point to mention these issues in order to make it clear that none of that really matters. If you’re able to endure these small problems, I can guarantee that you’ll find it the best Japanese grammar source available. At least, I couldn’t find any book, blog or YouTube channel in the English language that is so thorough and accurate.
After going over every single of her videos, I never felt so capable of understanding phrases in the wild. After translating any unknown words, the sentences’ meanings very frequently become clear to me, which is a completely different feeling than before. I started doing her recommended immersion techniques and, comparing with only two months ago since I last tried watching something with Japanese subtitles, the improvements are huge. By the way, if you subscribe to Netflix I also recommend the Language Learning with Netflix Chrome extension, very useful for going over line by line of the subtitles (not sure where you live, but here in Japan the anime catalogue is pretty decent).
Finally, I’m glad that you’re motivated to learn grammar. It’s very common for people to prefer to learn foreign languages without going deeply into grammar, which to me causes a huge time loss. Knowing grammar allows you to shortcut several breakthroughs that would take years to find by yourself!
In any case, whichever source you end up deciding, good luck with your studies!