(I'm not super advanced, but…) For me, it was understanding noun modification (how extra information comes before a noun, which is the reverse order, compared to English). For me, this has been the key to understanding longer texts and forming Japanese sentences in a more natural manner, instead of trying to directly translate word for word from English. I still have to "work backwards" for longer sentences, but I think it has really positively impacted my speaking, writing reading, and listening. (Though I'm trying not overuse this structure and make sentences sound super long and awkward.)
I learned this grammar point last year from Genki I, but I really internalized it when I was talking to a native speaker last month and I was trying to say something like "I have a friend that eats ramen every day." They just said 「毎日ラーメンを食べる友達がいます。」That was a light bulb moment for me. The other day my teacher showed a picture of someone wearing a blue coat and shivering, and was asking students to describe it. I immediately thought 上着を着ている人がいます and felt happy (maybe 上着を着ている冷たそうな人がいます would have been better?).
I'm curious if anyone has had any similar experiences of understanding a particular grammar point, maybe in a particular way, that really improved any combination of their speaking, writing, reading, and listening abilities.
by Altruistic-Mammoth