Formal possessives (cuyo, cuya)

To say "whose" in formal Spanish, use the relative adjective cuyo.

Cuyo connects a possessor to the thing they possess. It changes its form to match the gender and number of the possessed noun (the word immediately after it), not the possessor.

SingularPlural
Masculinecuyocuyos
Femininecuyacuyas

Notice how cuyo and cuya agree with the singular noun that follows them, regardless of who owns it:

This fortress, whose heritage we will protect, is very important.
Esta fortaleza, cuyo patrimonio protegeremos, es muy importante.
It is a historical building whose imposing facade you will like.
Es un edificio histórico cuya fachada imponente te gustará.

When the possessed noun is plural, use cuyos or cuyas:

The architect whose blueprints we will use designed this majestic castle.
El arquitecto cuyos planos usaremos diseñó este castillo majestuoso.
They will restore the historical monument whose statues are marble.
Ellos restaurarán el monumento histórico cuyas estatuas son de mármol.