Somebody, Something, Everybody

Greek's positive indefinites split cleanly into the ones that DECLINE and the ones that never change. κάποιος (“someone / some”) is a full -ος / -α / -ο word that agrees with its noun: κάποιος άνθρωπος, κάποια μέρα, κάποιο παιδί, and in the plural κάποιοι / κάποιες / κάποια (“some”). Its tonos never leaves κά-. όλος (“all, whole”) declines just as regularly and does two different jobs depending on where the article sits: όλη η μέρα is “the WHOLE day”, όλες οι μέρες is “ALL the days” - and note that the noun keeps its article, exactly as it does after a demonstrative. καθένας / καθεμία / καθένα (“each one”) declines too but has NO PLURAL, because “each one” is singular by definition; its genitive is καθενός. Against those stand three words that never inflect at all: κάτι (“something”), κάθε (“every”, always in front of a noun - κάθε μέρα) and the negative τίποτα. Nothing to learn about their forms; everything to learn about where they sit.