Grammar
Grammar is not something you should “study,” even though it is one of the three things that instructors teach when students are learning to speak, read, write, or listen to a new language. The same goes for vocabulary and pronunciation. I can give you a lesson in pronunciation, but your best bet is to practice speaking rather than pronouncing, and studying vocabulary won’t guarantee that you will use it or recognize it in real situations. Practice in Reading, listening, and conversation are better ways to acquire new vocabulary.