Cuyo is a relative adjective that means whose, of whom, or of which.
Like other Spanish adjectives, cuyo has masculine, feminine, singular,
and plural forms:
Cuyo is used to indicate the noun who or which is the object of
the clause that follows it. Cuyo is relative because it introduces a
relative clause, and an adjective because it must be followed by a noun.
Note that cuyo has a possessive value as well.
Es el chico cuyo padre murió.
It's the boy whose father died.
¿Dónde está el profesor cuyas clases comienzan a las ocho?
Where is the teacher whose classes begin at 8?
El escritor cuyos libros Ana quiere comprar vive en México.
The writer whose books Ana wants to buy lives in Mexico.
Esa casa, cuya puerta está abierta, es muy bonita.
That house, whose door (or "the door of which") is open, is very pretty.