"You must not forget."
Translation:Vi devas ne forgesi.
It could be.
But not every opposite distinction has to use "mal-" in Espperanto; there are some opposites that have individual words. (And a much larger number that have optional separate words in poetry, e.g. olda for maljuna or frida for malvarma -- the latter hardly ever used by itself outside poetry but whose derivative fridujo is very common!)
I know this old but I'm going to reply anyway. Esperanto was designed to be as simple as possible. Part of that should be not having an excessive amount of words for the same thing. I look at 'olda' the way I look at 'cheapo'. People created the word 'cheapo' as though taking the English word 'cheap' and putting an 'o' at the end made it valid Spanish. The correct word is 'barato', by the way. Cheapo means nothing. It's the same thing with 'olda'. Adding an esperanto ending to an english word shouldn't make it valid esperanto, when there's already a perfectly good word for it. Another example is 'malsanulejo' vs 'hospitalo'. Hospitalo was created simply by adding an -o ending to an english word. Malsanulejo literally means "not-healthy-person-place". Isn't that exactly what a hospital is? Adding a word for slang or words that can't already be conveyed with existing esperanto words is one thing. Adding duplicates words for the same thing completely defeats the stated purpose of esperanto. To be easy to learn.
That's kind of double -- "devi" is to be obliged to, and making that into a command is as if you said something like "You must be obliged not to forget".
What is being command is really about the forgetting - not about the being obliged to do something.
Perhaps an even better English translation for conveying the strangeness is "You have to must not forget."
@mizinamo I certainly support your interpretation and that's also how I try to use Esperanto. However, it's worth bearing in mind that not everyone makes/has made this distinction. PMEG has a good explanation here: http://bertilow.com/pmeg/gramatiko/gravaj_verboj/povi_devi_voli/neado.html#i-pks
Due to the above confusion in use, it might be better to use "ne bezonas" or "ne estas necese" to convey the meaning of "don't have to". Either that, or one could stick with "ne devi" and hope that it is understood correctly and that this interpretation catches on!