It's an adverb. I think this can be compared to the smell and taste expressions, where you have adjectives in English but we have adverbs in Swedish.
Det luktar illa 'It smells bad'
Det smakar gott 'It smells good'
Basically we ask 'How does she feel? How does it taste? How does it smell?' and then we answer the question 'how' with an adverb.
Yeah, for my part I (obviously) don't know Swedish well enough to know all the connotations of "mår".
Anecdotally, "She feels poorly" connotes acute, temporary, physical malaise, like coming down with a cold. "She is doing poorly" is more of a continuous thing, like having a rough time in life, or in a class, or with an ongoing sickness. If "Hon mår dåligt" can mean that as well, I'd say it's an acceptable translation. :)
Ok, so does "hur mår du" mean "how are you?" as in a general greeting, or does it just mean "how are you feeling?". Because in English I commonly greet people "how are you?", "how ya doing?" or "how's it going?", and the response would be "I'm doing good (or well if you want to be correct or formal)" if you are in a good mood, or "I'm doing bad (badly)" if not. Do Swedes say "hur mår du?" in this context, as a general greeting, or would they only say it if they really wanted to know how the person is, such as if they thought the person looked sick? Would Swedes think I was weird if I greeted them "hej, hur mår du?". Anyway, if "hur mår du?" does correspond to "how are you?", then I would think that "Jag mår bra" would be best translated as "I'm doing fine/good/well". But your answer that "hon mår bra" doesn't correspond to "She's doing fine" leads me to believe that I had misunderstood the usage of "hur mår du?". Sorry that was long, but maybe you could help clarify the usage of these phrases for me. Thanks so much
We do ask variations of such in similar ways, but not Hur mår du? specifically. I wouldn't use that unless I genuinely wanted to know.
Hon mår bra typically refers to a more thorough level of well-being than "She is going fine" does. While they can be translations (e.g. for a mother speaking about her child having had a cold), they usually are not.