Indefinites: κάποιος, κανείς, τίποτα
Someone, something, no one, nothing — and the double life of κανείς, which means “none” with δεν but “anyone” in a question.
The positive set
κάποιος (someone) and κάτι (something, invariable); μερικοί (a few); κάθε (each, invariable); όλος (all, agreeing); άλλος (other). Κάποιος agrees like an adjective: κάποια μέρα, κάποιο πρόβλημα.
The double life of κανείς and τίποτα
With δεν, negative meaning: Δεν ήρθε κανείς (nobody came), Δεν θέλω τίποτα (I want nothing). In a question, positive: Ήρθε κανείς; (did anyone come?), Θέλεις τίποτα; (do you want anything?). Same word, two polarities — context decides.
όλος and κάθε
όλος + article: όλη η μέρα (the whole day), όλοι οι φίλοι μου (all my friends). κάθε without article: κάθε μέρα (every day). Classic trap: “everyone” = όλος ο κόσμος.
Your turn
Choose the correct form.