How do you know when to use "은"/"는" vs. "이"/"가" ? I am a complete beginner with Korean. I can somewhat read hangul and I understand a few words and phrases but that's about it. In the book that I am using (which I got while I was in Korea), they say that "은"/"는" indicates a topic and "이"/"가" indicates a subject; however, I have never had topic and subject present to me as two separate concepts when discussing grammar before so this really confuses me. I've seen them both alone and and together in sentences but I still don't quite understand how they are used differently from one another. For example, what would be the difference between saying "책이많습니다" and "책은많습니다?" Are one of these simply wrong? If so, why? I'd greatly appreciate any help on the matter.