I introduced the Present Subjunctive to 2 of my students within the last week; first to my Spanish High School student and then to my Adult ESL student. Wow! Heavy duty grammar! I used to find it easier to teach Spanish grammar, in either English or Spanish, since I had to learn it formally and then incorporate it into my Spanish vernacular. I don't remember studying English grammar to the same extent, ever! At least until I took linguistics classes toward my Masters in ESL. However, I have always felt that learning these and other grammatical concepts in Spanish crept over to my English. I mentioned this to one of my other High School Spanish students and he agreed! He felt that the grammar he is learning, both from his teacher at school and from me, is helping his English!
In fact, my knowledge of Spanish grammar also helped me transfer the same concepts to other languages I have learned as well, including Italian and Hebrew.
More reason to study other languages, even if you don't become fluent.