Which one (cuál, cuáles)

You already know qué (what) and quién (who). Spanish has one more essential question word: cuál (which). Like quién, it changes with number:

Asking aboutForm
one thingcuál
more than onecuáles

Choosing one from a group

Use cuál and cuáles when the answer is a choice from a set of things: which one? which ones?

Which ones are your books?
¿Cuáles son tus libros?
Which ones are the house keys?
¿Cuáles son las llaves de la casa?

"What is your name?" — use cuál

English asks for names, addresses, and numbers with "what", but Spanish treats these as choices from many possible answers, so it uses cuál es. Saying ¿Qué es tu nombre? is a classic English-speaker mistake — Spanish never asks it that way.

Which is the new word?
¿Cuál es la palabra nueva?
What is your doctor's name?
¿Cuál es el nombre de tu doctor?
Which one is your family's house?
¿Cuál es la casa de tu familia?

Qué es asks for a definition

Use qué es only when you want an explanation of what something is:

What is a hospital?
¿Qué es un hospital?

Before a noun, use qué

Directly before a noun, Spanish prefers qué. (In Latin America you will also hear ¿cuál libro?, but qué + noun is always correct.)

What book do you need?
¿Qué libro necesitas?
What music do you listen to?
¿Qué música escuchas?