In theory, qué and cuál(es) are equivalent to what and which in English, respectively. Although this is true in most cases, there are a few exceptions.
¿Qué? feels like asking about something general. For instance, if we ask ¿qué quieres para comer? we're not thinking about a specific kind of food, we're referring to any kind of food you might think of.
On the other hand, when we ask ¿cuál?, a limited set of choices comes to mind. Going back to the example, if we knew there were only two possible meals available, like salad or pasta, then the question would be ¿cuál quieres comer?
The usage of these interrogative pronouns varies in a very special way specifically when we use them with the verb ser. Let’s see how.
When we use this combination, what we expect from the answer is the definition of something. Generally, the nouns used in these kind of questions are preceded by an indefinite article like un, una, unos, unas or demonstratives like eso, esa, esto, este, etc.
Cuál can be used to ask for specific information when combined with the verb ser.
Yes. There are three ways we can use qué + ser to get specific information:
Qué + ser can be used to ask for specific information just like cuál, but it uses the construct lo + adjective to produce a very general noun -like cosa (thing) or parte (part)- that doesn't need to be stated explicitly, but rather just described with an adjective. This process is called nominalization (sustantivación in Spanish).
In this example, the word cosa isn’t necessary (although it is in English) but if we decided to include it, then we would have to use cuál. Crazy! We know!
In a situation where Merly and Michael are already talking about movies, she asks him:
She knows they’re talking about movies, so she doesn’t need to use that word explicitly. However, if she decides to use it, then the question is:
Let’s make this clearer with two more examples:
Another case where it’s possible to use qué + ser to ask for specific information is when it is used with the relative pronoun lo que.
But if we use more specific relative pronouns such us el que, la que, los que, las que we need to use cuál + ser.
For instance, if Andrea shows Merly a group of books, she could ask:
Let’s compare them one more time:
Speaking about a specific group of songs:
When the question starts with a preposition, it’s possible to use qué + ser to get information. Let’s see:
We can use qué and _cuál before a noun and both will be correct, but there is a clearly perceptible change in the sensación (feeling) that it produces to native speakers. Let’s see:
With qué, it is implied that there is an indefinite amount of books (like "what").
With cuál, it is implied that there is a limited selection of books (like "which").
Remember to use the plural form cuáles for plural nouns!
In questions where verbs other than ser are used, qué means what and cuál(es) means which:
So just remember: you need to pay special attention when using qué and cuál with the verb ser. If you keep these rules in mind, you'll find that it's really not that confusing after all!