Numbers and case

1. When a noun is used with a number the case of the noun will be determined by the number used.
2. With the number 1, and all other numbers that end in 1 (except 11), the noun will be in the nominative case.
   1 год
   21 неделя
   31 час
   101 письмо
3. With the numbers 2-4, and all other numbers that end in these digits (except 12, 13, 14), the noun will be in the genitive singular case.
   2 года
   23 недели
   34 часа
   102 письма
4. With all other numbers, including 11-19 and numbers that end in 0, the noun will be in the genitive plural case.
   5 лет
   26 недель
   12 часов
   117 книг