In my opinion, the lessons jumped in to the Genitive case too early. The singular and plurals are pretty straightforward, generally speaking. Learning the Nominative Case, the Accusative Case, the Dative case, and the Locative case are also straightforward. I would recommend just using the hints and getting through the Genitive case as quickly as possible so you can move on in the lessons, then actually come back to try and learn the Genitive case once you are much further along in Russian.
The verb to have and genotove case!!!!!!?????
У женщины есть яблоко (affirmative phrase) У женщины нет яблока (negative phrase)
When I have the word "нет" the next noun must be in the genitive case???
Because I'm 1st phrase the word remained яблоко
But in the 2nd phrase the word changed to яблока
From the notes on Basics 2:
" У A есть X ~ by A there is an X → A has an X
The owner [i.e. subject] is in the Genitive case "
And bear in mind нет is the 'opposite' of есть. For completeness, from the notes on Genitive 1:
" 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 "
вода is whenever you're talking about water in the Nominative case (when it's the subject of the sentence, grammatically speaking). воды is the genitive version.
Btw, you can look up all forms of the word with wiktionary (https://en.wiktionary.org/wiki/%D0%B2%D0%BE%D0%B4%D0%B0#Russian). It's what I'm using!
Russian vowels come in pairs. ы and и both are roughly i sounds but ы sounds a bit different, the tongue is a bit further back which makes it sound sort of a cross between i and uh.
Because ы and и are pairs, ы is used after normal (hard) consonants, whereas и is used when the base form of a verb ends with -ь or another soft vowel (-я, -е, -ё, -ю), or with a consonant where it is hard to pronounce an ы after it (к, г, х, ш, щ, ж, ч).