That really is a factual issue with Brazilian Portuguese pronunciation (especially this approximation to the Rio de Janeiro dialect, however robotic it may sound at times): words ending in a vowel + L combination usually turn into diphthongs, so "mal" does end up sounding a lot like "mau" (regular people make the same mistake you just did).
That said, the only possible option here on structural, logical terms is an adverb (since you're qualifying the verb "trabalhar", i.e. how he works), so only "mal" could ever fit the bill for the sentence.
It's clear from the discussion that the sentence means the manager's work is bad. The question is how to translate it, since he "works badly" is simply not idiomatic in English, though there's nothing grammatically wrong with it. Should Duo be made to accept translations that represent what a native English speaker would say to convey the meaning of the Portuguese sentence? That would be good for non-native English speakers who are using the program to improve their English. Examples could be "The manager does poor work" or "The does his job poorly." Other solutions?
I think it is because "hardly works" means he does not work the time is required or is quite difficult to make him work. On the other hand "works badly" means he doesn't work they way it has to be worked, and that's what "trabalha mal" means.
In spanish we have "trabaja mal" and has the same escence than the portuguese one, but "mal trabaja" do means "hardly works". I do not know if you can say "mal trabalha" but I think it would be the correct translation for your answer.
This helped me a bit. If somebody can clarify what I got from this is pretty much mau/bom are opposites of each other and mal/bem are also opposites of each other. Mal is used for badly/barely instead of mau which is used for just the word bad. Again, anyone feel free to chime in and correct! :D
You are correct on pairing bom/mau and bem/mal. The former ones are adjectives and therefore refer to a substantive. The latter ones are adverbs and therefore refer to verbs or adjectives, mal can actually mean bad, as long as it is used as an adverb like this exercise that uses badly
Bad is an adjective (describes nouns) and badly is an adverb (describes verbs, and sometimes adjectives – in this case how he works).
Adverbs can usually be identified in English by the suffix ly (added at the end) such as safely, nicely, wonderfully and of course, usually, But not always as well for instance is also an adverb. Adverbs describe how an action is done or performed. They are careful (describes they; careful people); they cut carefully (describes how they cut).
All that said, I personally would use poor and poorly in many sentences rather than bad and badly. He has poor taste in clothes but, the milk tastes bad. She works poorly but, she feels badly about that.
But also just the negation, he does not work well is often used.
Since you brought it up... 'She feels badly' would really mean that her sense of touch is not working correctly. 'She feels bad' refers to the emotional state. I love semantics! The educational system in America is so poor that I think most Americans couldn't tell you what an adverb is, let alone use them correctly or identify the differences in those sentences. Most non-native English speakers seem to have a far better understanding of grammar and parts of speech than Americans, especially on this site.