What type of Spanish do you teach? There are many Spanish-speaking countries, each with their own dialect, so studying a specific one as the main focus while learning to distinguish the others seems the way to go.
Since I was born in Spain and have lived in it for more than 20 years, I can teach European Spanish as a native, while also helping you understand how Latin American variants of it work when it comes to verbs and the like.
I'm currently struggling a lot trying to memorize "kanji" (Japanese characters), as I seem to forget how they're written or even what they mean. What's a good way of keeping them in my head?
One of the hardest parts of the Japanese language is learning the "Jōyō Kanji", which are the 2136 characters necessary to have a real grasp of texts and the language itself, as they're used in the media and the press. However, many foreigners have stumbled upon this wall before and have devised different memorization methods to get past it. As a student of Japanese, I can attest to the efficiency of some of those methods, while rejecting some "traditional" ones based on my own experience.
What are good ways of distinguishing between British English and American English, so that I don't mix them up while writing or speaking?
You will mostly notice the differences that arise when speaking of a certain topic by how does each dialect use a noun or a grammar rule that distinguishes it from other types of English. Take into account examples when it comes to spelling. In British English, words like "colour" or "labour", which lose their "u" when spoken in American English (thus, they become "color" and "labor"). The same goes for verb conjugations, different words altogether or cultural differences unique to each dialect.