"They work hard."
Translation:Eles trabalham duro.
I'm not sure if I understand your question, but you linked "hard" to "they"? If I got it, you wrote "eles trabalham duros", but they meant it was a hard work... When it's the way they DO something it's always singular, but attention, when it's the way something is at the moment it follows the "something". And in portuguese we add "mente" at the end of the words.
(I really hope that it helps, I don't know the right words in english for a few grammar stuff, sorry)
Actually you are right and the other responses reference the way people think about it. Technically it is an adjective being used as an adverb -- something that happens with certain fairly frequent adjectives, muito being the first to convert. But its form is masculine singular (duramente, the more formal and standard way of making adjectives, note, has feminine form because mente historically was fs) in this kind of use, even if there is no ms noun to reference it to. These colloquials occur only with intransitive v. Don't confuse them with verbs that take an adjective complement… < sigh > complex grammar…. sorry.