Genitive Plural Endings

Masculine Nouns:
Most hard-stem masculine nouns (including those ending in velars k and g) take -ów (dom → domów, ptak → ptaków). Most soft or hardened stems take -i or -y (use -i after soft consonants like ć, ń, ś, ź; use -y after hardened consonants like cz, sz, rz, ż) (gość → gości, lekarz → lekarzy). Nouns ending in -c, -dz, or -j are unpredictable: some take -ów (chłopiec → chłopców, kraj → krajów), while others take -y (miesiąc → miesięcy, pieniądz → pieniędzy).

Feminine & Neuter Nouns:
These usually take a zero ending, meaning you drop the final vowel of the nominative singular (kobieta → kobiet, drzewo → drzew, mieszkanie → mieszkań).

Important Spelling Rules for the Zero Ending:

• Fleeting 'e': Some nouns have a mobile e in the genitive plural; learn their stem alternation (matka → matek, okno → okien). Note that feminine nouns ending in -ka or -ga almost always trigger a fleeting 'e' or a vowel shift when they take the zero ending (książka → książek, noga → nóg).
• Vowel Shifts: The stem vowel 'o' often changes to 'ó' (słowo → słów), and 'ę' changes to 'ą' (ręka → rąk, święto → świąt) when the ending is dropped.

Exceptions and Special Cases:

• Feminine Consonant Stems: Feminine nouns that already end in a consonant in the nominative singular take -i or -y (rzecz → rzeczy, noc → nocy). This includes the large category of nouns ending in -ść, which take -i (miłość → miłości).
• Feminine Nouns in -ja, -ia, or -ea: These often do not take a zero ending. Instead, they take -i, -ii, or -ji (restauracja → restauracji, babcia → babci, linia → linii). Some native soft-stem nouns permit zero forms: jaskinia → jaskiń retains final ń, but suknia → sukien has mobile e and final n. Suknia also allows sukni. Babcia allows both babci and babć; do not rule out its zero form.
• Masculine Nouns in -a: Masculine nouns ending in -a typically take the standard masculine -ów ending (kolega → kolegów, artysta → artystów), with the notable exception of mężczyzna, which takes the zero ending (mężczyzn).
• Soft-Stem Neuter Nouns: Neuter nouns ending in -dzie or -że often take -i or -y (narzędzie → narzędzi), while those ending in -cie or -rze typically take a zero ending (zajęcie → zajęć, morze → mórz).
• Neuter Nouns in -um: Loanwords ending in -um take the masculine ending -ów (muzeum → muzeów).
• Neuter Nouns in -ę: These expand their stems to end in -ąt or -on (zwierzę → zwierząt, imię → imion).
• Nouns in -anin: Masculine personal nouns ending in -anin drop the -in and take either a zero ending or -ów (Rosjanin → Rosjan, Amerykanin → Amerykanów).
• Core Irregulars: Some very common nouns have highly irregular genitive plural forms: rok → lat, człowiek → ludzi, dziecko → dzieci, przyjaciel → przyjaciół.

Note: A common misconception is that the genitive plural distinguishes masculine-personal nouns. It doesn't! That happens in the accusative plural, which borrows these genitive forms for masculine people.

Soft stems do not invariably choose -i/-y: uczeń → uczniów (student), widz → widzów (spectator). The ending must be learned with the noun. Genitive plural is not mechanically predictable from the final consonant alone.