"We do not have butter."
Translation:У нас нет масла.
I present to you, one of the finer links I've been given (by Theron126): http://starling.rinet.ru/cgi-bin/morphque.cgi?flags=endnnnnp
You can input any Russian word that you like, and it will open up a new tab with all the meanings as well as - and that's what's important here - all the shapes in all the cases. (There was something about letters like ё that it might have trouble with. Just use е instead.)
And by cases, I mean, the Russian cases of its grammatical system. So nominative, genitive, dative, etc. etc.
If the noun is the subject of the sentence, you write it in the nominative case (plural or singular, depending). You can then look at the table and go "Ah, it should be масло". However, if we are dealing with the genitive case, it should be масла. Fortunately, I've had some help from Google and identified this sentence as accusative instrumental - so why's it in this lesson? - so that means it is still масло.
Here's a link to an overview of what each case addresses: http://masterrussian.com/aa071600a.shtml
I hope that doesn't muddle things up further because now I'm wondering if it shouldn't be genitive case, which means the Russian is wrong. (Except it can't be wrong, because I'm not a native speaker or an expert, so there's no way I'd be right about it being wrong. .. Right?)
«Ма́сла» IS genitive in «У нас нет ма́сла». After «нет» we use genitive.
As for the question about choosing «ма́сла» or «ма́сло»: Basically, we can't tell them apart in speech (except in some dialects). If you get this sentence in a listening exercise, you need to understand the sentence, understand the role of the noun in the sentence and write the correct form based on your understanding. Yes, it does mean that you have to do some 'conversion' from spoken Russian into written one, but this is the case for almost all the languages (compare English "its" and "it's").
Please see my answer (and Shady_arc’s correction) here: https://www.duolingo.com/comment/11557601$comment_id=11557823
No, we never use «не е́сть» in such a context.
«Не е́сть» almost always means «not to eat», so your sentence sounds like a rule: "At our place, no eating butter" ≈ "At our place, eating butter is forbidden!". You often use infinitives for such rules (e.g. по траве́ не ходи́ть = no walking on the grass, не паркова́ться = no parking).
Having consulted with my college professor, as well as a colleague who is a former Russian translator, there's not technically a rule saying you can't, which is to say that the sentence isn't actually wrong, however the omission есть doesn't change the meaning of the sentence, and is the form used in Russia itself. Rather than counting the sentence wrong they should simply point out that есть isn't necessary and shouldn't be used, but I don't think duolingo's software back end can do that for entire words, just minor typos.
I'm afraid you must have misunderstood your professor and colleague.
I'm pretty sure «у нас есть не масла» would never be used to say 'We don't have butter" (it could mean "What we have aren't butters/oils" if you put stress on the last syllable, масла́). I'm a native speaker from Belarus, but I'm pretty sure this is true in Russia too.
Очень спорно! "Сливочное масло" для бутерброда! Потому что, масло (тем более во множественном числе - ударение на последнюю букву) может быть разным. Намазка на бутерброд (в виде масла) - одно наименование (слова масла)из десятков различных по свойствам продуктов! "У нас нет сливочного масла" - дословно!