There are tips and notes for each lesson, only visible in the web app. https://www.duolingo.com/skill/ru/Genitive-Case---1
Tips and notes
In Russian “I have” is expressed by «У меня (есть)» structure. The owner is in the Genitive case.
"The of-case". It is one of the most universal cases. How do you make the forms? Here is the regular pattern:
A zero ending means that the word ends in a consonant or a soft sign (which is just a way to show the final consonant is "soft"). In the Nominative singular, a Russian word can only have the following endings: а, я, о, е, ё ornothing ("zero ending").
GENITIVE OF NEGATION
If you use «нет» to say that there is "no" something or you do not have it, the object is always in Genitive:
У меня́ есть я́блоко → У меня́ нет я́блока
Здесь есть рюкза́к → Здесь нет рюкзака́.
"of" (possession): яблоко мамы = mom's apple"of" (amount): чашка чая, много чая = a cup of tea, a lot of tea
A huge number of prepositions requires this case. Yes, «у меня есть», «У неё есть» only use «меня» and «неё» because «у» wants Genitive.
For он, она and оно Genitive doubles as a non-changing possessive "his", "her", "their": его, её, их.
initial «н» is used for him/her/them with the majority of prepositions (doesn't affect possessives)
A little side note: some nouns of foreign origin are indeclinable. It means that all their forms are the same. Foreign nouns that end in о/е become like that (кофе, метро, радио, резюме), as well as all nouns that do not fit into Russian declension patterns (see above).
This includes female names that end in anything other than А or Я. A few -ь-ending names are an exception (Любовь and Biblical names like Юдифь).
So, all of the following names are automatically indeclinable: Маргарет, Мэри, Элли, Дженни, Рэйчел, Натали, Энн, Ким, Тесс, Жасмин.
I AM AWAY
Russian also uses the Genitive to state that someone is "away", "not there": Мамы сейчас нет. In English such use would correspond to "There is no mom at the moment", or even "There is no me now". We are not hard on that particular construction in the course, but it is important to know it all the same.
Added bonus: when a verb directly acts on a noun, the noun is called a direct object and is in Accusative. In Russian, only -а/-я feminine nouns have a unique form for it. Others just reuse Genitive or don't change the word at all (Nominative)
Russian uses.... let's call it "consistent" negation. It means that in negative sentences you are required to use "nothing" instead of "anything", "nowhere" instead of "somewhere" and so on. Let's meet the first of these pronouns:
У меня ничего нет. = I don't have anything.Она ничего не ест. = She doesn't eat anything.
You'll also notice that, unlike standard English, Russian has no rule against using double negatives.
I haven't heard "neta", but saying "netu" (не́ту) in place of "net" (нет) is pretty widespread. It's not considered standard, though, so it's only used in colloquial speech, but not in formal writing.
Note that "netu" (не́ту) is only used when talking about absence of something. As a general-purpose negative answer, only "net" (нет) is used.
«We don't have juices» would be «У нас нет со́ков».
"Hi, may I have some apple juice please." waiter: "Sorry, we don't sell juiceS here."
— Здра́вствуйте, мо́жно мне я́блочного со́ка, пожа́луйста?
— Извини́те, у нас тут не́т со́ков.
@mquarmoc - Сока is the genitive singular form of сок. The word нет uses genitive case (it is an example of the "nonexistence or absence" which requires genitive case). You can use singular with it (нет компьютера - there is no computer) or plural (нет компьютеров - there are no computers).
Here's a page that shows other examples and uses of the genitive case in action: https://www.clozemaster.com/blog/russian-genitive-case/
@Languagecr10 - This is a really simple explanation and you'll find it doesn't cover 100% of situations.
But, "have not" is generally past tense ("I have not seen the movie yet").
If we're talking about present tense, then we need "do/does not" ("I do not have the juice").
Of course there are phrases in the present tense that use "have not" (well, "haven't", like, "I haven't [got] the slightest idea").
The sentence definitely says «у нас нет сока», where did you find the sentence with «о»?