The Genitive is used to express the meaning 'some' (Дђйте мнљ молокђ Give me some milk'). This usage contrasts with the Accusative much as English 'some' contrasts with 'the' (Дђйте мнљ молокћ Give me the milk').
However, most #-declension nouns denoting non-countable things (like cheese, chocolate, tea, etc.) have the special case ending -у/-ю, called the Partitive case, to express 'some'. The Partitive case is a dying category: some speakers rarely use it. You don't have to use it when you speak, but you must learn to recognize it.
Here are a few nouns of this sort: сџп, сІр, сђхар, шоколђд, and чђй. The words бћрщ and хлљб are exceptional, in that they have no Partitive case forms. Note that a masculine noun with this ending looks like a feminine noun with an Accusative ending:
Передђй, пожђлуйста, икрџ. (Acc. fem.) Pass the caviar, please.
Дђй, пожђлуйста, икрІ. (Gen.) Give me some caviar, please.
Передђй, пожђлуйста, сІр. (Acc.) Pass the cheese, please.
Дђй, пожђлуйста, сІру. (Part. masc.) Give me some cheese, please.
Передђй, пожђлуйста, хлљб. (Acc.) Pass the bread, please.
Дђй, пожђлуйста, хлљба. (Gen.) Give me some bread, please.
The Partitive case is used after quantity words (скћлько 'how much', мнћго 'a lot', немнћго 'a little bit' мђло 'not much'). (Words having no Partitive forms are in the Genitive case after quantity words).
Скћлько у тебї икрІ? (Gen.) How much caviar do you have?
У менї мђло икрІ. (Gen.) I don't have much caviar.
Скћлько у тебї сІру? (Part.) How much cheese do you have?
У менї мђло сІру. (Part.) I don't have much cheese.
Скћлько у тебї хлљба? (Gen.) How much bread do you have?
У менї немнћго хлљба. (Gen.) I have a little bit of bread.
The use of the Partitive varies from speaker to speaker, particularly when the noun follows another noun specifying the exact quantity (including нљт):
У нђс нљт сІра. (Gen.) [or: сІру (Part.)] We don't have any cheese.
Дђйте мнљ килогрђмм сІра. (Gen.) [or: сІру (Part.)] Give me a kilo of cheese.
Ї купќла фџнт сІра. (Gen.) [or: сІру (Part.)] I bought a pound of cheese.
In Genitive contexts where quantity is not involved, the Partitive is not used:
цвљт сІра (Gen.) the color of cheese
ікспорт сІра (Gen.) cheese export