I put "The man who is running is fat" because that is how an English speaking person would say this. It was counted incorrect.
I would correct and say that's how YOU say this. English has many dialects, even within the states. I put "The running man is fat" and that doesn't sound weird for my English. That said, yours doesn't sound weird either, so yes, both being allowed would be preferable, but it isn't that one is wrong, rather both are right.
Because semantically it requires a different syntactic construction to represent this.
This lesson is about modifiers, ergo, the key modifier here is the adjective "running". It wants to show you how to adjectivalize a gerund specifically. Further, this is in the present simple tense. What your sentence accomplishes, while still more or less similar, is a demonstration of the present progressive tense, which by now we have not learned yet.
Because the predicate of this sentence is 뚱뚱합니다 "is fat." "The fat man" would be 뚱뚱한 남자가. Remember, the modifier comes BEFORE the word it modifies in Korean.
DuoLingo MODs, must you perpetuate the extremely bad habit Koreans have of calling people who aren't a 00 or a 0 fat? It's rude! This fat, running woman is requesting that the sentence be changed to something more encouraging! Thank you.
I put "the fat man is running"... Surely that's close enough? It delivers the same point in an equally correct way, grammatically speaking. I understood why it was marked wrong, but ahh.
One way of understanding this is that you mixed up what is new and old information. In "the fat man is running," it's already understood that there is some fat man, and you're telling the listener that he is running. In "the running man is fat," it's the other way around.
Of course, context helps in cases like this, but those two sentences definitely do convey different meanings.
"The fat man is running" is continous tense while "the running man is fat" is present.
That would be: 뚱뚱하는 남자가 달리고 있어요.
는 is not added to a descriptive verb (adjective)--뚱뚱한. Also, Koreans often express the continuous tense with the simple present.
The key to this sentence is what pluckingstrings has pointed out. The focus of the original sentence is on a man running, who happens to be fat. You could change fat to any of a number of descriptions-- The man running is slow. The man running is tired. The man running is healthy. The man running is ugly. etc.
"The fat man is running." focuses on a fat man who happens to be running, which you could change to a number of activiities. The fat man is swimming. The fat man is eating. The fat man is belching. The fat man is floating. etc.
The korean sentence intself is wrong. It is aupposed to be, 남자의 달리는 뚱뚱합니다 which means "The man's leg is fat"