"I am not a book."
Translation:저는 책이 아닙니다.
I think we use 는 and 은 as the topic particle. To make a plural topic, then we should use (in some cases), the plural particle 들 + 은. Also, I think that the pluralization would not be always necessary.
남자들은 (Men) 아이들이 (children) 아닙니다 (are not). This would be a possible translation for: Men are not children, I am not too sure (as a general sentence, the pluralization would not be necessary in Korean), and I still think that Korean people use some different forms to translate the English sentence.
Also, I am not really sure if the plural particle is necessary if we put the plural pronoun, then 우리는 (We) 아이들이 (children) 아닙니다 (are not).
I hope it helps. =)
EDIT: I have put the incorrect marker, the correct is called topic used to add emphasis, contrast, or to limit what is being talked about. Now it is fixed, and I add the reference from the Notes - To Be (by Soedori)
| Limited topic | 저는 여자입니다. (I am a woman.) | Irrelevant of anyone else, I am a woman. (May imply that someone else might be as well.) |
| Contrasting topic | 저는 여자입니다. (I am a woman.) | Unlike the others, I am a woman. |
은/는 can be used with general statements as well because you only want to talk about the notion as a group, and nothing else.
Korean Notes Repository (===under construction==): https://www.duolingo.com/comment/24352980