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무엇은 is a logical guess since 뭐 is short for 무엇, but it is wrong. The reason 뭔 doesn't mean 무엇은 is likely because 무엇은 is a phrase almost never used in practice. If you are asking something phrasing it like English "what is ...?", you'd say 무엇이 or its short form 뭐가, not 무엇은. This is because 은/는 either states some idea you have or present something ...


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Despite the apparent etymology, "뭔" doesn't retain its meaning from the contraction. The synonym suggested by Standard Korean Language Dictionary is "무슨" (colloquially). 뭔 「관형사」 ‘무슨’을 구어적으로 이르는 말.


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