Přivlastňovací zájmeno svůj

English "his" is ambiguous. In "Petr talked to Pavel about his car", whose car is it? Czech does not allow the doubt. When the owner is the subject of the same clause, Czech uses the reflexive possessive svůj; jeho, její, jejich then point to someone else.

One word for every person

Sentence	Whose key?
Petr hledá svůj klíč.	Petr's own
Petr hledá jeho klíč.	another man's, say Pavel's
Jana hledá svůj klíč.	Jana's own
Jana hledá její klíč.	another woman's
Rodiče prodávají svůj byt.	the parents' own flat
Hledám svůj klíč.	my own

In the third person the choice changes the meaning: Petr políbil svou ženu is a happy marriage; Petr políbil jeho ženu is a scandal. In the first and second person both words point to the same owner, so Hledám můj klíč is not wrong, but svůj is the usual choice; můj there sounds emphatic.

English "their" after "everyone" is also svůj: Každý má svůj názor. So is "own": Mám svůj vlastní pokoj.

Forms

Svůj declines exactly like můj and agrees with the thing owned: svůj klíč, svou (svoji) chatu, své (svoje) auto, svého bratra, se svými dětmi. Svou and svoji, své and svoje are equal standard variants.

Where it cannot go

Svůj refers back to a subject, so it cannot be part of the subject: Petr a jeho bratr přišli, not *svůj bratr. And it normally looks at the subject of its own clause: in Jana řekla, že Petr prodal své auto the car is Petr's.

Watch the cottage invitation: Jana pozvala Petra a jeho sestru na svou chatu. Jeho sestru is Petr's sister; svou chatu is Jana's cottage. English manages here with "his" and "her"; with two men in the sentence it could not, and Czech still would.

Svůj in sayings

Každý svého štěstí strůjcem: everyone forges their own luck. Svůj k svému, "each to his own people", was a nineteenth-century slogan urging Czechs to buy from Czech shops. And Je svůj describes a person who goes their own way, one of the few places svůj stands without a noun.