Mai = ever, or never, depending on context.
Non dimenticare mai ... don't ever forget/never forget
ora più che mai ... now more than ever
If you get stuck on one of these words, you might like to visit Context Reverso. It's a searchable database of translated texts (including subtitles for films and TV shows), and it can give you lots of examples of how a given phrase has been translated in the past. Great site.
Have a sunshine day.
Duolingo doesnt teach you stock phrases. It's not a phrase book. It teaches you how to construct sentences and apply logic and grammar rules to use the language for your own needs. So you might not ever need to say "Never use this sugar" but it the sentence nicely demonstrates how to use the infinitive form of the verb, demonstrative articles, the double negative, where to put the noun, etc etc.