koī and kuch are indefinite pronouns/quantifiers. As pronouns, koī is used for animate singular ("someone") and kuch for animate plural and inanimates ("something"). As quantifiers/adjectives koī is used for singular count nouns and "kuch" for mass nouns and plural count nouns. koī takes the form kisī in the oblique. The form kaī is a paucal equivalent to koī, being used in the context of "several" or "a few" things. kuch can also act as an adverb, qualifying an adjective, meaning "rather". koī preceding a number takes the meaning of "about, approximately". In this usage it does not oblique to kisī.