Working? As in being employed by Duolingo? In general the moderators of a course are doing this voluntarily, so they only get to work on issues when they have the time for it. I guess on the larger courses, like Spanish and German, they have employees doing research in order to improve quality. But most of the ones directly employed by Duolingo are engineers or designers.
Well, no one is working in this discussion forum - it's just us students helping each other, I'm afraid.
However, the course developers have made a huge effort recently and added reported alternative answers to masses of questions. They just haven't got round to this one yet, by the look of it. Patience is rewarded - keep reporting!
1) Use the usual subject prefixes ("u-" sg., "m-" pl.)
2) Drop the tense marker (as for positive polite commands)
3) Add the negative marker “-si-”
4) If the verb end is “-a”, change it to “-e”
If the verb end is “-i” or “-u”, do not change it.
Usinunue! - You shouldn’t buy it! (you sg.)
Usichelewe! - Don’t be late/delay! (you sg.)
Msiondoke! - You shouldn’t leave! (you pl.)
Msisahau! - Don’t forget! (you pl.)
Msirudi! - Don’t return! (you pl.)