grammar.pronouns.relative-cuyo
Cuyo: 'whose', agreeing with the thing owned
Cuyo/cuya/cuyos/cuyas means 'whose' and agrees with what is possessed, not the possessor: la empresa cuyos empleados...
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Explication
Cuyo means 'whose' and behaves like a possessive bridge between a noun and what it owns. Crucially, it agrees with the thing possessed, not the possessor: el autor cuya novela ganó ('the author whose novel won') — cuya because novela is feminine, even if the author is male. Forms: cuyo, cuya, cuyos, cuyas.
It is formal and almost entirely written. In conversation, speakers route around it with a relative clause: el autor que escribió la novela que ganó, or la empresa de la que no recuerdo el nombre.
The classic error to avoid is *que su: *el autor que su novela ganó is wrong — that is exactly the gap cuyo fills. So 'the company whose name I forget' is la empresa cuyo nombre olvido, never *la empresa que su nombre olvido.
Exemples
It's a company whose name I don't remember.
Région: global
The house whose windows face the sea is that one.
Région: global
It's a project whose results aren't visible yet.