сыр

The genitive is sometimes used to indicate "some". This is called "partitive" genitive. Most nouns use genitive for partitive, but this noun has a special "partitive" genitive ending -у, as well as сахар - сахару, чай - чаю. E. g. Питер купит сыру. "Peter will buy some cheese." Πитер кyпит килограмм сыру "Peter will buy a kilogram of cheese." However, the ordinary genitive may also be used in such sentences as the latter: Πитер кyпит килограмм сыра.