сыр

The genetive is sometimes used to indicate "some". This is called "partitive" genetive. Most nouns use genetive for partitive, but this noun has a special "partitive" genetive 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.