Hi, 1. The Accusative case in Russian indeed marks a direct object, but this is not a single case that does it. For example, the Dative case does that as well. It is not only about the sentence structure (Subject - Object - Verb), it is about the meaning, i.e. a situation (!) 2. One of the situations that the Dative Case marks is addressing (I added a dash just for convenience): сказать мам-е/друг-у - to tell to mother/friend. написать мам-е/друг-у (to write to mother/friend) прочитать мам-е/другу-у (to read to mother/friend) объяснить мам-е/друг-у (to explain to mother/friend) Compare with Accusative: сказать речь - to tell a speech (you are not addressing to speech) написать книг-у, письм-о - to write a book/letter (you are not writing to a letter) прочитать книг-у, письм-о - to read a book/letter объяснить теорем-у, правил-о - to explain a theorem/rule 3. One of the meanings that Genitive case has, is "partitive" i.e. you take a part of the whole. In English you normally say "some" in this situation. Words take different suffixes than they normally do with the Genitive case (-ы instead of -у, -ю instead of -я). It is still called Genitive case, but some scholars prefer to treat it as a separate case. Anyway, there're not so many words that take that case. Я выпил вод-ы - I drank some water vs. Я выпил всю вод-у - I drank all the water (in a cup/bottle etc.) Я выпил ча-ю - I drank some tea vs. Я выпил весь чай - I drank all the tea Hope this helps!

the noun in the accusative case does not point to an object that performs an action (this is a direct complement). Я куплю машину, я возьму друга с собой, я выпью газировки, я ищу тетрадку, стол имеет 4 ножки, тетрадь имеет 96 листов, кружка имеет ручку. I buy/get/have/find what or whom without this noun the proposal is incomplete (I buy what? I drink what? I look for what?) он сказал мне we specify the recipient, this word is not a direct complement он сказал мне (мне indirect object of action)

Такие глаголы, как сказать, выпить, говорить, в первую очередь требуют винительного падежа, т.е. Accusative. Например, сказать речь, сказать слово (то, что можно произнести), выпить воды, читать книгу, писать письмо. Но! многие из этих слов употребляются и с другими падежами. Например, сказал (кому?) мне (кому предназначалось слово, кому предназначалась речь) (это дательный, не Genitive).

Hi there, I'm also a Russian learner, and I always thought that it was just something you have to memorize, like how we say "he told me", versus "he said TO me". In Russian, "show me" is "покажи мне". So maybe try and think that if we use the word "to" with the verb in English, like explain TO me, show TO me, write TO me, say TO me...then there is a good chance that the verb will be followed by the dative case (мне) in Russian. I don't know if this strategy is always correct, but I think it can help! (For example, "help me" in Russian is "помоги МНЕ"...we don't say "help to me!" in English, but it is implied... make what you will of it!

An addition to Irina's explanation: with some verbs it is as simple as 'some water' vs. 'THE water'. Я выпил воду means excatly: I drank the water.