I'm having a hard time trying to guess the meaning of the third 은 in this sentence:
남산은 놉은 산은 아니지만 참 아름답습니다.
I know that this sentence means "Namsan is not a tall mountain, but it's really beautiful".
I know that the first 은 marks the subject. The second 은 is an adjectival suffix, needed when the adjective accompanies a noun within the same syntactic function. But I can't figure out what the third 은 means. It seems to mark the object of the verbal root 아니 ("not to be"). But, if I'm not mistaken, the idea of "(not) being sth" does not need a particle correlating that verb to its object.
Am I missing something?