It is important to understand that in Russian the case is what effectively gives a word its meaning in the sentence. In Russian language there are 6 cases:
1. The nominative (Именительный падеж) expresses the subject or predicate by answering to: Who? What? – Кто? Что? It is an initial, dictionary form of noun.
Девочка плавает – The girl is swimming
Who is swimming? The girl (a noun answers the question “who”).
2. The accusative (Родительный падеж) expresses the direct object by answering to: Whom? What? – Кого? Что?
He is reading a book – Он читает книгу
What is he reading?- A book (answers the question “what”) and the ending of the nominative form of the noun “книга” changes to a “книгу”.
3. The dative case (Дательный) denotes that something is given or addressed to the person or to the object and it answers the question : To whom? To what? – Кому? Чему?
I go to see my friend – Я иду к моему другу.
Whom am I going to? To my friend (answers the question “to whom”) and to a nominative form of the noun “друг” is added the ending “у” and it become “другу”.
4. The genitive case (Винительный) is used to show that something (or somebody) belongs or refers to something (or somebody) else, (very similar to English “of” ) and answers the question: of whom? (whose) of what? – Кого? (Чей) Что?
край стола – the edge of the table.
The edge of what? (answers the question “of what”) and the ending of the nominative form of the noun “стол” changes to “стола”.
5. The instrumental case (Творительный падеж) denotes an instrument that helps to make the action and answers the question: Кем? Чем? – by whom? With (by) what?
I am eating with the spoon – я ем ложкой.
What am I eating with? (answers the question “with what?”) and the ending of the nominative form of the noun “ложка” changes to a “ложкой”.
6. The prepositional case (Предложный падеж) is used to designate a place, person or a thing that is object of speech or though. This case is used always with a preposition: about whom? О ком? О чём? Где? – About what? Where?
Чашка чая стоит на столе – the cup of tea is on the table.
Where is the cup of tea? (answers the question “where”) and the ending of the nominative form of the noun “стол” changes to a “столе”.