I can use English quite well, but most of my "book knowledge" about grammar, like the kind of thing the SAT might ask about, actually comes from my French classes. That's were I learned the names of different tenses (present-progressive, past-conditional, etc) and such.
*Edit* - At the same time, however, I still think it's silly to say "this is what is right, and you are wrong if you do anything differently" because, as I said, the language has always changed, and will continue to change. Sure the first few new adopters of a new word or grammatical construction will seem strange or stupid to others, but once it's become widely adopted it's part of the language and isn't wrong anymore.
Fun fact: using "they" for a non-gendered singular pronoun has only recently become "wrong".