The "Zero Ending"
To form the genitive plural of most feminine nouns ending in -а and neuter nouns ending in -о, you simply drop the final vowel. Because no new letter is added, this is known as a "zero ending."
If dropping the final vowel leaves a difficult consonant cluster (especially for words ending in -ка), Russian inserts a helping vowel to make pronunciation easier:
- -о- is usually inserted between hard consonants. For example, банка becomes банок and коробка becomes коробок.
- -е- is inserted after soft consonants or "husher" letters like ш and ж. For example, девушка becomes девушек.
If the word does not end in a difficult cluster, you simply drop the vowel. For example, the neuter noun яблоко becomes яблок.
Smooth Endings
If the noun ends in a single consonant before the -а or -о, or if the remaining consonants flow easily, you don't need to insert any vowels.
For instance, слива becomes слив, груша becomes груш, and ягода becomes ягод. Notice that even though груша has a husher letter (ш), it doesn't need an inserted vowel because there is no consonant cluster before the ending.
Soft Consonants and the Letter -я
When a noun ends in -я or has a soft sign in its stem, the genitive plural often uses an inserted -е- or ends in a soft sign (-ь).
- The neuter noun письмо drops its -о and inserts an -е- to become писем (replacing the soft sign).
- Feminine nouns ending in -ня like вишня drop the final vowel and insert an -е- to become вишен.
- Feminine nouns ending in -ля like туфля drop the final vowel, insert an -е-, and keep a soft sign (-ь) to become туфель.