Translation:Books are interesting.
would it always be a general concept? if this sentence is with the topic particle .. from my standpoint it could also be "the book" first mentioned? .. else on subsequent mentions with the subject particle -가/이 .. 책이
so then why does 은 make it plural? is its use as a plural for general statements?
The Korean does not use plural, but in English we tend to use plural in general statements. Also, without the plural marker the Korean could be singular or plural. With the plural marker, then it cannot be viewed as singular.
That is the topic marker. It marks the noun as what we are talking about. It could be the subject of the sentence, but the topic doesn't have to be. https://www.duolingo.com/skill/ko/basics-1/tips-and-notes
It can be ommited. The speaker is making a general statement about books if a specefic book is not indicated in the conversation.
이 책은 재미있습니다.
That is not correct because the topic marker is being used. The English word "subject" refers to the grammar "job" of the noun and this can be a topic, or something else in the sentence can be a topic. https://www.duolingo.com/skill/ko/basics-1/tips-and-notes
It mostly means 'fun'
what would change the sentence to be "the book is interesting" vs. "books are interesting."
Is the topic marker then used more for generalities and the subject marker for something specific?
That would mean "books are interest", as in the noun "interest". 책은 재미있습니다 means "books are interesting", with 재미있습니다 meaning the adjective, "interesting".
It can be singular or plural, but there is a plural marker if you need to talk about a specific plural noun. https://www.duolingo.com/skill/ko/basics-1/tips-and-notes Scroll up, the moderator explains that the plural marker is not used for generalities.
I feel like while interesting is the correct translation. Saying they are cool or awesome should also be acceptable translations
what makes 재미있습니다 be "interesting" and 재미없습니다 be boring? what differenciates them? how can they have opposite meaning while being so much alike when written