"We need to slice mushrooms for the salad."
Translation:Нам надо нарезать грибы для салата.
Okay I'm going out on a limb but на was previously used in the context of "for lunch", so I would theorise that "нарезать грибы на салат" might mean that the salad consists of those mushrooms and nothing more, and that the sentence has the nuance that slicing mushrooms is the act of creating a salad.
In contrast, the version with для means that the mushrooms augment the salad.
Yes, but careful: usage of prepositions depends on context, there's often not a one-to-one correspondence. для - 'for,' 'for the benefit of' Это для тебя. = This is for you. Мы были там две недели = We were there (for) two weeks. (but don't use для to indicate duration) на - 'on' Книга на столе. = The book is on the table. на - 'at, to' an event Она пошла на концерт. She went to the concert. Он поехал в Мосвку на неделю. = He went to Moscow for a week (indicates time following the action)
Careful – 'the' doesn't have a direct equivalent in Russian, and it's not connected with the choice of для or на, which depends on context. For example...
Это для тебя. = This is for you.
Это для студентов. = This is for (the) students.
Они пошли на концерт. = They went to a/the concert. (depending on the context)
Hardly a scientific study, but for what it's worth: a quick Google search gives these results: "грибы в салат" - 26,600 (though many are in the context of putting mushrooms into the salad)
"грибы на салат" - 70,700
"грибы для салата" - 5,880
In any case, prepositions often can't be translated directly from one language into another; they're best learned in context. And perhaps this is an example that one shouldn't stress over, anyway; Google finds no examples of 'нарезать грибы в салат/на салат' - and the only 3 examples with нарезать грибы для салата are from... Duolingo pages. :P
Надо and нужно are impersonal (subjectless) expressions that take the dative - literally, "to us it's necessary to cut.." Russian often uses these to express feelings, possibility, necessity, and permission.
Мне надо работать. = I need to work. Тебе трудно? = Are you having a hard time? Им нельзя говорить по-английски. = They're not allowed to speak English.
There's a short video about it here:
same question here. Maybe надо is more about the necessity of doing something (think of "I must -or have to- cut mushrooms to make salad") and нужно about the necessity of having something, aka "I need [to have] mushrooms to make salad"), but I am basically just pulling this out of my .... hat ;-)
so, I got some unexpectedly quick reply when I put the question on the Russian Grammar youtube channel (on a 6-year old video, nonetheless!). You can check it here https://www.youtube.com/watch?v=PR5iOYybOdc&lc=z22ezhzzzvbvglhdsacdp43afmpzm3fwundsqvrdbvxw03c010c.1578247458624212&feature=em-comments (my commenter name there is "Jörg Löhken").
The short of it is that there is really not much of a difference, aka it should be accepted. Surprisingly, the explanation that I attempted in my prior comment was actually useful, but for all intents and purposes and in this particular case, there should not be any distinction, so you were totally right with your question, Duolingo should, at some point, accept нужно as well. They are just a bit slow about it, but let's keep in mind that most of the people there are probably volunteers, and if you take just 5 people asking like us every day, for every language, that is already a sh...load of answers to give and complaints to process.
adverb is a non-inflected part of speech. Adverbs do not have gender, case or number. They never change – except qualitative adverbs in -о or -e which have a comparative and a superlative degree. ‧ russianlearn.com/grammar/category/formation_of_adverbs ‧