The use of the interrogative pronouns qué (what) and cuál (which) in Spanish can sometimes be confusing for English speakers. There are certain rules about the proper use of these words, but, as usual, sometimes grammar doesn't quite catch up with how things work in the real world.

Witht that being said, our best advice is always to listen more and practice more. So let's see some examples of how questions with qué (what) and cuál (which) are used in real context!

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Cuál vs Qué: The Choice Factor

Generally speaking, we can say that cuál (which) is used to ask about the identity of a person or a thing in a group. Cuál is a question about choice. The pronoun qué is for questions about identity as well, but it's used in direct questions rather than in ones that involve making a choice. Here's an example where the question cuál (which) is used before a verb:

¿Pero cuál es el peor trabajo?

But which is the worst job?

Caption 40, 75 minutos - Del campo a la mesa

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It would be really odd to hear someone saying ¿Qué es el mejor trabajo? It's rare, but possible, because making this substitution alters the meaning of the question completely. It's easier to understand the distinction if we compare what kind of answer these two questions would likely get:

¿Pero cuál es el peor trabajo? / But which is the worst job?
- El peor trabajo es el de recogedor de basura. / The worst job is that of a garbage hauler.

¿Pero qué es el peor trabajo? / But what is the worst job?
-El peor trabajo es el menos deseable. / The worst job is the least desirable.

Now, let's check the following example. Here cuál is used before a noun:
 

¿Pero cuál juego les gusta más?

But which attraction do you like the most?

Caption 36, Guillermina y Candelario - El parque de diversiones

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This is a great example, because when used before a noun, cuál is equivalent to qué. So the following is also correct:

¿Pero qué juego les gusta más?
But which attraction do you like the most?

We believe it's actually better to see it the other way around: sometimes qué (what) is also used to ask about choices. It happens in English as well, right?
 

¿Qué color te gustaría?

What color would you like?

Caption 50, Raquel y Marisa - Agente del concesionario

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But most of the time it's just not right to use qué to ask about choices. Let's see an interesting phenomenon. In Spanish, you can use the preposition de (of) to introduce a group of things from which to pick a choice. And you can use cuál (which) before this preposition:

¿Cuál de esos países te gustó más?

Which one of those countries did you like most?

Caption 86, Cleer - Hobbies

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You can also use qué (what) before de (of), but then you wouldn't be asking someone to pick a favorite country from a list of countries (the ones the other person visited), you would be asking something else: 

¿Qué de esos países te gusto más? 
What [aspect] of those countries did you like the most?

It's very useful to imagine possible answers to both questions:

¿Cuál de esos países te gustó más? / Which one of those countries did you like most?
- Italia y Francia. / Italy and France.

¿Qué de esos países te gustó más? / What aspect of those countries did you like most?
- Su rica cultura y deliciosa comida. / Their rich culture and delicious food. 

But, if you place the preposition de (of) before and not after qué or cuál, you can again use them as equivalents! Why? Because now they are both right before a noun.

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¿De qué sabor quieres? = ¿De cuál sabor quieres?
What flavor do you want? = Which flavor do you want?

We'll continue to explore the use of cuál (which) and qué (what) in future lessons.