"A singer is not fun."
The first one has “charm” as a noun, so while we say “ “A singer is charming.” There is a topic marker on “singer”, the Korean actually says “Regarding the singer, there is charm.” or “Regarding the singer, charm is there.” which acknowledges the subject particle on “charm”. https://www.duolingo.com/skill/ko/basics-1/tips-and-notes
There is a totally different construction for the next sentence. The adjective is actually a descriptive verb. It is not separate from the verb. https://www.duolingo.com/skill/ko/Descriptive-Verbs/tips-and-notes
I wonder why we have to use an adjective at all. I wish we could just say “Singers do not amuse.” but I guess we would always say who they are not amusing. I wonder how you could say “Singers do not amuse me.” Oops, that is a different verb. So “a singer is not fun.” is not the same thing. Maybe it really should be “funny” instead of “fun”. Maybe it is entertaining, but perhaps it is a sad song?
Could it also be phrased "A singer has charm"? In which case, "singer" is, in a manner of speaking, the "owner" of the "charm"? Which then explains the respective markers (because according to the Course Notes, you use "은/는" for the "owner" of something and "가/이" for the thing being "owned").
Thank you for the very helpful distinction between nouns and adjectives though. That has definitely made Korean sentence structure (at least, what we're learning at this stage) a bit clearer for me.
When I see a new word translated twice with different English words that don't seem to go together to me, I try to hold them up together in my mind & see if/where they overlap. A mental Venn diagram. So like, 재미 can mean fun and also interesting. I think it must be sort of, "engaging" or "captivating", because that's where I think fun and interesting meet. Enjoyable, specifically in a way that holds your attention.
재미 should (almost) never translate to funny. I only stress this because my students confuse fun and funny all the time. This could also be translated as "the singer is not interesting" (interesting as in capturing my attention vs. intellectually interesting [흥미롭다]).
A/the singer is not funny = 가수는 웃기지 않아. / 가수는 우습지 않아.
So you don't want to put the topic marker? https://www.duolingo.com/skill/ko/basics-1/tips-and-notes