The phrase, “Learn some grammar” is technically correct grammar, because the subject, even though it is not initially stated in the sentence, is implied to be you. The predicate, however, is clearly seen in the word, “learn,” and in its extended form, “Learn some grammar,” with you taking the active and grammar taking the passive. The phrase, “Learn some grammar” is also stated as a command, which, similarly to the commands “Go” or “Come,” does not require a subject in their sentences as it is implied. So don’t go around throwing out false corrections, please. Learn some grammar is indeed correct.