WHEN TO USE CUYO / CUYA
Cuyo / cuya / cuyos and cuyas all mean whose. By now you will be accustomed to the fact that there is a masculine and feminine option and that they exist in both the singular and plural form.
Whose can relate to the possession of something by either a person or a thing and is not a Possessive Pronoun as you might think but actually a Relative Adjective. The form of cuyo you use must agree with the thing being possessed and not with the person or thing that is the possessor.
EXAMPLES
La niña cuyo gato es negro. The girl whose cat is black.
El hombre cuya hija es estudiante. The man whose daughter is a student.
La mujer cuyos libros son rojos. The woman whose books are red.
El niño cuyas hermanas son guapas. The boy whose sisters are pretty.
El gato cuya pata es blanca. The cat whose paw is white.