Translation:We need to wash the apples for the salad.
Нам надо may be so common in Russian that leaving off нам isn't strange to a native-speaker.
My reaction was to mentally translate it as "one must/needs to" which in English is often expressed as a kind of general "we" - kind of like the Royal "We", except it's lower-case, and could be termed the common "we". It's a substitute for the more formal "one must".
It's also a substitute for "it", as in "it is necessary". I don't know why Duo doesn't accept "it is necessary", but Duo certainly should - unless my first thought is really strong in Russian - that a native-speaker naturally hears the unspoken нам. I tried it today, and it was rejected, so I reported it. The "correct" answer Duo gave me was "1 must wash the apples for the salad" - literally, the numeral "1", not the general pronoun "one".
It's really a shame that Duo leaves so much out which would help people understand the language at a basic level. I don't understand why the moderators do that. With a couple tweaks, Duo could be a stellar language program. As it is, it gets a solid C+/B-, less if they charged money.
We need to wash the apples for salad - is rejected for leaving out 'the' in front of salad. Not an unusual inappropriate use of 'the', although it would be used if it referred to a salad previously identified. Like, "I am going to school", as distinct from "I am going to the school".
This is not a general statement (I was previously wrong on this, until I consulted my Russian tutor), so "One needs to wash apples for salad" should not be accepted. The reason is that a general statement, being about more than one situation, would require the imperfective verb, thus "Надо мыть яблоки для салата.".
Yandex translates the Russian text as "I need to wash the apples for the salad", and Google as "We need to wash apples for the salad". DL should also accept "The apples should be washed for salad": see the question https://forum.duolingo.com/comment/11845484.
Duo needs to include the lessons/tips for the app as well since it's already online. Moreover, this is a pretty nasty one, i also wanted to go with "one needs to", or "the apples need to be" ... Without some proper explanation this is just learning by through error and seeing the answer since all we have is our intuition... However even still I'm left with more questions