"I have never worked so hard before."
Translation:Ik heb nog nooit zo hard gewerkt.
It isn't clear to me when to use "nooit" and when to use "nog nooit". My answer was marked wrong because I omitted the "nog" (Ik heb nooit zo hard gewerkt). In other sentences in this exercise, "nooit" is used on its own (Ik heb nooit gezegd dat ik jouw tuin niet leuk vind). Should my answer be correct, or is there a rule that I'm missing?
"nog nooit" = never before. Something is currently happening that has not happened in the past. If this is clear from context (like in this sentence), you can leave out "before" in English, but you wouldn't leave out "nog" in Dutch.
In the 2nd example I gave, the "nog" was omitted in the correct answer but seems to me the difference is a subtle one. Perhaps this is a way of explaining it: When you are trying to convey that until this point, it has not been the case, use nog nooit. When the use of "never" is a denial, like in the 2nd example, the "nog" is not needed.