Special cases - the noun-count rule

After 1, the noun stays in its normal singular form: један сто, једна сестра. After 2, 3 or 4, the noun takes a special counting form - for masculine and neuter nouns this looks like the genitive singular (сто → стола), for feminine -а nouns it looks like the nominative plural (сестра → сестре). After 5 and up, the noun takes the genitive plural (сто → столова, сестра → сестара).

For compound numbers, only the last digit decides the form: двадесет један сто (21) behaves like 1, двадесет два стола (22) behaves like 2–4, двадесет пет столова (25) behaves like 5+ - however large the whole number is. The teens (11–20) are the one exception: they always take the 5+ genitive-plural form, regardless of what they end in.