I am native Russian speaker, familiar with most aspects of Russian grammar and syntax, and being able to explain all the rules and peculiarities, as I learned it throughout all 10 years in my school in Russia. I also know what it is to learn a new language, being a speaker of English and Chinese myself.
Aside from that I took math courses through Calculus and higher, tutoring myself some higher level math, while I was an undergraduate student in UW-Madison. I think I had great math professors, from whom I learned not only the theory and problem-solving techniques, but also way of teaching math.