The rules for the Genitive plural nouns are rather complex. In short, the genitive plural is formed with the help of -ов, -ев, -ей endings. Some nouns drop the last vowel (рыба -> рыб), while others change to a different word used in the plural (год -> лет, человек -> людей).
The choice of the ending depends on gender of the noun (masculine, neuter, feminine), stress paterns, or which letters the "dictionary form" ends in (consonant, soft sign, etc.) The "dictionary form" is the Nominative singular. Here are the guidelines:
-ов ending
Pick -ов for all masculine nouns ending in a consonant other than ж,ч,ш,щ Also, review the rules below and if they conflict with this one then apply them instead.
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студент (dictionary form) -> студентов (Genitive plural)
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час -> часов
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дом -> домов
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вопрос -> вопросов
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мальчик -> мальчиков
-ев ending
Pick -ев for all masculine nouns edning in -й and the unstressed -ец suffix. Note how -ец transforms into -ц in the plural.
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случай -> случаев
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герой -> героев
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палец -> пальцев
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американец -> американцев
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but not: конец (because the -ец suffix is stressed) -> концов (see the rule above)
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and not: отец (because the -ец suffix is stressed) -> отцов
-ьев ending
Pick -ьев for nouns whose Nominative plural form ends in -ья
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брат/братья -> братьев
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лист/листья -> листьев
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дерево/деревья -> деревьев
-ей ending
Pick -ей for masculine nouns ending in -ь and ж,ч,ш,щ
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путь -> путей
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писатель -> писателей
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день -> дней (letter 'e' is omitted so ден- becoms дн-)
Pick -ей for all feminine nouns ending in -ь
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жизнь -> жизней
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ночь -> ночей
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дверь -> дверей
Pick -ей for neuter nouns ending in -e
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море -> морей
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поле -> полей
Dropped last vowel
The last vowel is dropped in neuter nouns ending in -o
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дело -> дел
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слово -> слов
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место -> мест
The last vowel is dropped in feminine and masculine nouns ending in -а, -я
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голова -> голов
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нога -> ног
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партия -> партий (pronounced /партийа/ and /a/ is dropped)
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армия -> армий (pronounced /армийа/ and /a/ is dropped)
Nominative singular = Genitive plural
A few nouns have the same Genitive plural as the Nonimative singular
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раз
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солдат
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глаз
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партизан
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человек -> человек or людей
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чулок
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сапог