Translation:Don't eat unhealthy food.
The adjective has more than one syllable and it modifies the noun. Read here for more information https://resources.allsetlearning.com/chinese/grammar/Structural_particle_%22de%22
By the same standard, "Don't eat unhealthy things" is also a double negative.
There are double negatives that are grammatically incorrect (e.g. "The pilot can't find no place to land") and then there are double negatives that accurately convey the intended meaning and are grammatically correct (e.g. "Not a day goes by when I don't think about you.") I believe that my sentence falls under the second category.
From: englishplus.com › grammar "Healthy or Healthful? In formal English, things are healthful (i.e., good for one's health). People or other creatures are healthy (i.e., in a state of good health)." It is true that "healthy' is used frequently in informal speech when 'unhealthful' is the correct word but that does not make it correct grammar. Our Chinese lesson requires us to learn correct grammar in word order because that is a mark of an educated writer, and it is a pity that it does not encourage us to use correct grammar in English in our answers.
I see. I wasn't aware you were approaching this from a standpoint of prescribed grammar. That's all well and good then.
I'm of the school of thought that language is not used because it is correct, but rather is correct because it is used. That's the very reason why every language is constantly changing and evolving. I also have heard AND read "healthful" in use so infrequently that if I were to see it as the default recommended translation, it would sound a bit unnatural to me. It's not a part of the shared experiences (or in other words, culture) held by me and others in the community I grew up with. I'm not sure if I'm the only one or not, but perhaps this is evidence that the word "healthful" is slowly dying out.
Dear Andy - you are right that 'healthful' is slowly dying out in usage. I also recognize that languages change over time, but worry about losses in precision, as in this case. In the language community in which I live and read, healthful is still used, but Duolingo has apparently decided against healthful and unhealthful so I will have to 'go with the flow'. best wishes