Either way there are many other resources, I'm sure some of them are even better than the one I linked. The real answer is that no matter what resource you learn from you have to practice consistently (just like every other part of the language). Using the grammatical concepts in sentences as you learn them tends to help, but no matter what you do, you'll forget stuff sometimes. Just keep working!
Renshuu has its own grammar curriculum called Japanese Basics, which you can find in the menu. There are vocab schedules that tie in with it, so you're intended to study vocabulary and grammar together as to never be thrown an example that's above your level.
I will say though that Japanese is very different from most other world languages, so in the beginning, wrapping your head around the grammar and sentence structure may be difficult, and it's up to to to see which methods work best. For me, I had to go through multiple sets of grammar explanations and beginner audio lessons and youtube playlists all explaining the same material in different ways before it really sank in, but spending all that time getting comfortable with beginner grammar made it very easy and quick to move through more advanced grammar when I was ready. Be patient with yourself, and don't be afraid to try out different methods and resources to find what helps you the most.
